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perlfaq5 (1)


perlfaq5 - Files and Formats


Please see following description for synopsis


Perl Programmers Reference Guide                      PERLFAQ5(1)

     perlfaq5 - Files and Formats

     This section deals with I/O and the "f" issues: filehandles,
     flushing, formats, and footers.

  How do I flush/unbuffer an output filehandle?  Why must I do
     (contributed by brian d foy)

     You might like to read Mark Jason Dominus's "Suffering From
     Buffering" at .

     Perl normally buffers output so it doesn't make a system
     call for every bit of output. By saving up output, it makes
     fewer expensive system calls.  For instance, in this little
     bit of code, you want to print a dot to the screen for every
     line you process to watch the progress of your program.
     Instead of seeing a dot for every line, Perl buffers the
     output and you have a long wait before you see a row of 50
     dots all at once:

             # long wait, then row of dots all at once
             while( <> ) {
                     print ".";
                     print "\n" unless ++$count % 50;

                     #... expensive line processing operations

     To get around this, you have to unbuffer the output
     filehandle, in this case, "STDOUT". You can set the special
     variable $| to a true value (mnemonic: making your
     filehandles "piping hot"):


             # dot shown immediately
             while( <> ) {
                     print ".";
                     print "\n" unless ++$count % 50;

                     #... expensive line processing operations

     The $| is one of the per-filehandle special variables, so
     each filehandle has its own copy of its value. If you want
     to merge standard output and standard error for instance,
     you have to unbuffer each (although STDERR might be
     unbuffered by default):

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             my $previous_default = select(STDOUT);  # save previous default
             $|++;                                   # autoflush STDOUT
             $|++;                                   # autoflush STDERR, to be sure
             select($previous_default);              # restore previous default

             # now should alternate . and +
             while( 1 )
                     sleep 1;
                     print STDOUT ".";
                     print STDERR "+";
                     print STDOUT "\n" unless ++$count % 25;

     Besides the $| special variable, you can use "binmode" to
     give your filehandle a ":unix" layer, which is unbuffered:

             binmode( STDOUT, ":unix" );

             while( 1 ) {
                     sleep 1;
                     print ".";
                     print "\n" unless ++$count % 50;

     For more information on output layers, see the entries for
     "binmode" and "open" in perlfunc, and the "PerlIO" module

     If you are using "IO::Handle" or one of its subclasses, you
     can call the "autoflush" method to change the settings of
     the filehandle:

             use IO::Handle;
             open my( $io_fh ), ">", "output.txt";

     The "IO::Handle" objects also have a "flush" method. You can
     flush the buffer any time you want without auto-buffering


  How do I change, delete, or insert a line in a file, or append
     to the beginning of a file?
     (contributed by brian d foy)

     The basic idea of inserting, changing, or deleting a line
     from a text file involves reading and printing the file to
     the point you want to make the change, making the change,

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     then reading and printing the rest of the file. Perl doesn't
     provide random access to lines (especially since the record
     input separator, $/, is mutable), although modules such as
     "Tie::File" can fake it.

     A Perl program to do these tasks takes the basic form of
     opening a file, printing its lines, then closing the file:

             open my $in,  '<',  $file      or die "Can't read old file: $!";
             open my $out, '>', "$" or die "Can't write new file: $!";

             while( <$in> )
                     print $out $_;

        close $out;

     Within that basic form, add the parts that you need to
     insert, change, or delete lines.

     To prepend lines to the beginning, print those lines before
     you enter the loop that prints the existing lines.

             open my $in,  '<',  $file      or die "Can't read old file: $!";
             open my $out, '>', "$" or die "Can't write new file: $!";

             print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

             while( <$in> )
                     print $out $_;

        close $out;

     To change existing lines, insert the code to modify the
     lines inside the "while" loop. In this case, the code finds
     all lowercased versions of "perl" and uppercases them. The
     happens for every line, so be sure that you're supposed to
     do that on every line!

             open my $in,  '<',  $file      or die "Can't read old file: $!";
             open my $out, '>', "$" or die "Can't write new file: $!";

             print $out "# Add this line to the top\n";

             while( <$in> )
                     print $out $_;

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        close $out;

     To change only a particular line, the input line number, $.,
     is useful. First read and print the lines up to the one you
     want to change. Next, read the single line you want to
     change, change it, and print it. After that, read the rest
     of the lines and print those:

             while( <$in> )   # print the lines before the change
                     print $out $_;
                     last if $. == 4; # line number before change

             my $line = <$in>;
             $line =~ s/\b(perl)\b/Perl/g;
             print $out $line;

             while( <$in> )   # print the rest of the lines
                     print $out $_;

     To skip lines, use the looping controls. The "next" in this
     example skips comment lines, and the "last" stops all
     processing once it encounters either "__END__" or

             while( <$in> )
                     next if /^\s+#/;             # skip comment lines
                     last if /^__(END|DATA)__$/;  # stop at end of code marker
                     print $out $_;

     Do the same sort of thing to delete a particular line by
     using "next" to skip the lines you don't want to show up in
     the output. This example skips every fifth line:

             while( <$in> )
                     next unless $. % 5;
                     print $out $_;

     If, for some odd reason, you really want to see the whole
     file at once rather than processing line-by-line, you can
     slurp it in (as long as you can fit the whole thing in

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             open my $in,  '<',  $file      or die "Can't read old file: $!"
             open my $out, '>', "$" or die "Can't write new file: $!";

             my @lines = do { local $/; <$in> }; # slurp!

                     # do your magic here

             print $out @lines;

     Modules such as "File::Slurp" and "Tie::File" can help with
     that too. If you can, however, avoid reading the entire file
     at once. Perl won't give that memory back to the operating
     system until the process finishes.

     You can also use Perl one-liners to modify a file in-place.
     The following changes all 'Fred' to 'Barney' in inFile.txt,
     overwriting the file with the new contents. With the "-p"
     switch, Perl wraps a "while" loop around the code you
     specify with "-e", and "-i" turns on in-place editing. The
     current line is in $_. With "-p", Perl automatically prints
     the value of $_ at the end of the loop. See perlrun for more

             perl -pi -e 's/Fred/Barney/' inFile.txt

     To make a backup of "inFile.txt", give "-i" a file extension
     to add:

             perl -pi.bak -e 's/Fred/Barney/' inFile.txt

     To change only the fifth line, you can add a test checking
     $., the input line number, then only perform the operation
     when the test passes:

             perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

     To add lines before a certain line, you can add a line (or
     lines!)  before Perl prints $_:

             perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

     You can even add a line to the beginning of a file, since
     the current line prints at the end of the loop:

             perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

     To insert a line after one already in the file, use the "-n"
     switch.  It's just like "-p" except that it doesn't print $_
     at the end of the loop, so you have to do that yourself. In
     this case, print $_ first, then print the line that you want
     to add.

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             perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt

     To delete lines, only print the ones that you want.

             perl -ni -e 'print unless /d/' inFile.txt

                     ... or ...

             perl -pi -e 'next unless /d/' inFile.txt

  How do I count the number of lines in a file?
     (contributed by brian d foy)

     Conceptually, the easiest way to count the lines in a file
     is to simply read them and count them:

             my $count = 0;
             while( <$fh> ) { $count++; }

     You don't really have to count them yourself, though, since
     Perl already does that with the $. variable, which is the
     current line number from the last filehandle read:

             1 while( <$fh> );
             my $count = $.;

     If you want to use $., you can reduce it to a simple one-
     liner, like one of these:

             % perl -lne '} print $.; {'    file

             % perl -lne 'END { print $. }' file

     Those can be rather inefficient though. If they aren't fast
     enough for you, you might just read chunks of data and count
     the number of newlines:

             my $lines = 0;
             open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
             while( sysread $fh, $buffer, 4096 ) {
                     $lines += ( $buffer =~ tr/\n// );
             close FILE;

     However, that doesn't work if the line ending isn't a
     newline. You might change that "tr///" to a "s///" so you
     can count the number of times the input record separator,
     $/, shows up:

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             my $lines = 0;
             open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
             while( sysread $fh, $buffer, 4096 ) {
                     $lines += ( $buffer =~ s|$/||g; );
             close FILE;

     If you don't mind shelling out, the "wc" command is usually
     the fastest, even with the extra interprocess overhead.
     Ensure that you have an untainted filename though:

             #!perl -T

             $ENV{PATH} = undef;

             my $lines;
             if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
                     $lines = `/usr/bin/wc -l $1`
                     chomp $lines;

  How do I delete the last N lines from a file?
     (contributed by brian d foy)

     The easiest conceptual solution is to count the lines in the
     file then start at the beginning and print the number of
     lines (minus the last N) to a new file.

     Most often, the real question is how you can delete the last
     N lines without making more than one pass over the file, or
     how to do it without a lot of copying. The easy concept is
     the hard reality when you might have millions of lines in
     your file.

     One trick is to use "File::ReadBackwards", which starts at
     the end of the file. That module provides an object that
     wraps the real filehandle to make it easy for you to move
     around the file. Once you get to the spot you need, you can
     get the actual filehandle and work with it as normal. In
     this case, you get the file position at the end of the last
     line you want to keep and truncate the file to that point:

             use File::ReadBackwards;

             my $filename = 'test.txt';
             my $Lines_to_truncate = 2;

             my $bw = File::ReadBackwards->new( $filename )
                     or die "Could not read backwards in [$filename]: $!";

             my $lines_from_end = 0;
             until( $bw->eof or $lines_from_end == $Lines_to_truncate )

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                     print "Got: ", $bw->readline;

             truncate( $filename, $bw->tell );

     The "File::ReadBackwards" module also has the advantage of
     setting the input record separator to a regular expression.

     You can also use the "Tie::File" module which lets you
     access the lines through a tied array. You can use normal
     array operations to modify your file, including setting the
     last index and using "splice".

  How can I use Perl's "-i" option from within a program?
     "-i" sets the value of Perl's $^I variable, which in turn
     affects the behavior of "<>"; see perlrun for more details.
     By modifying the appropriate variables directly, you can get
     the same behavior within a larger program. For example:

             # ...
             local($^I, @ARGV) = ('.orig', glob("*.c"));
             while (<>) {
                     if ($. == 1) {
                             print "This line should appear at the top of each file\n";
                     s/\b(p)earl\b/${1}erl/i;        # Correct typos, preserving case
                     close ARGV if eof;              # Reset $.
             # $^I and @ARGV return to their old values here

     This block modifies all the ".c" files in the current
     directory, leaving a backup of the original data from each
     file in a new ".c.orig" file.

  How can I copy a file?
     (contributed by brian d foy)

     Use the "File::Copy" module. It comes with Perl and can do a
     true copy across file systems, and it does its magic in a
     portable fashion.

             use File::Copy;

             copy( $original, $new_copy ) or die "Copy failed: $!";

     If you can't use "File::Copy", you'll have to do the work
     yourself: open the original file, open the destination file,

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     then print to the destination file as you read the original.
     You also have to remember to copy the permissions, owner,
     and group to the new file.

  How do I make a temporary file name?
     If you don't need to know the name of the file, you can use
     "open()" with "undef" in place of the file name. In Perl 5.8
     or later, the "open()" function creates an anonymous
     temporary file:

             open my $tmp, '+>', undef or die $!;

     Otherwise, you can use the File::Temp module.

             use File::Temp qw/ tempfile tempdir /;

             my $dir = tempdir( CLEANUP => 1 );
             ($fh, $filename) = tempfile( DIR => $dir );

             # or if you don't need to know the filename

             my $fh = tempfile( DIR => $dir );

     The File::Temp has been a standard module since Perl 5.6.1.
     If you don't have a modern enough Perl installed, use the
     "new_tmpfile" class method from the IO::File module to get a
     filehandle opened for reading and writing. Use it if you
     don't need to know the file's name:

             use IO::File;
             my $fh = IO::File->new_tmpfile()
                     or die "Unable to make new temporary file: $!";

     If you're committed to creating a temporary file by hand,
     use the process ID and/or the current time-value. If you
     need to have many temporary files in one process, use a

             BEGIN {
             use Fcntl;
             my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
             my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;

             sub temp_file {
                     local *FH;
                     my $count = 0;
                     until( defined(fileno(FH)) || $count++ > 100 ) {
                             $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                             # O_EXCL is required for security reasons.
                             sysopen my($fh), $base_name, O_WRONLY|O_EXCL|O_CREAT;

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                     if( defined fileno($fh) ) {
                             return ($fh, $base_name);
                     else {
                             return ();


  How can I manipulate fixed-record-length files?
     The most efficient way is using pack() and unpack(). This is
     faster than using substr() when taking many, many strings.
     It is slower for just a few.

     Here is a sample chunk of code to break up and put back
     together again some fixed-format input lines, in this case
     from the output of a normal, Berkeley-style ps:

             # sample input line:
             #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
             my $PS_T = 'A6 A4 A7 A5 A*';
             open my $ps, '-|', 'ps';
             print scalar <$ps>;
             my @fields = qw( pid tt stat time command );
             while (<$ps>) {
                     my %process;
                     @process{@fields} = unpack($PS_T, $_);
             for my $field ( @fields ) {
                     print "$field: <$process{$field}>\n";
             print 'line=', pack($PS_T, @process{@fields} ), "\n";

     We've used a hash slice in order to easily handle the fields
     of each row.  Storing the keys in an array means it's easy
     to operate on them as a group or loop over them with for. It
     also avoids polluting the program with global variables and
     using symbolic references.

  How can I make a filehandle local to a subroutine?  How do I
     pass filehandles between subroutines?  How do I make an
     array of filehandles?
     As of perl5.6, open() autovivifies file and directory
     handles as references if you pass it an uninitialized scalar
     variable.  You can then pass these references just like any
     other scalar, and use them in the place of named handles.

             open my    $fh, $file_name;

             open local $fh, $file_name;

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             print $fh "Hello World!\n";

             process_file( $fh );

     If you like, you can store these filehandles in an array or
     a hash.  If you access them directly, they aren't simple
     scalars and you need to give "print" a little help by
     placing the filehandle reference in braces. Perl can only
     figure it out on its own when the filehandle reference is a
     simple scalar.

             my @fhs = ( $fh1, $fh2, $fh3 );

             for( $i = 0; $i <= $#fhs; $i++ ) {
                     print {$fhs[$i]} "just another Perl answer, \n";

     Before perl5.6, you had to deal with various typeglob idioms
     which you may see in older code.

             open FILE, "> $filename";
             process_typeglob(   *FILE );
             process_reference( \*FILE );

             sub process_typeglob  { local *FH = shift; print FH  "Typeglob!" }
             sub process_reference { local $fh = shift; print $fh "Reference!" }

     If you want to create many anonymous handles, you should
     check out the Symbol or IO::Handle modules.

  How can I use a filehandle indirectly?
     An indirect filehandle is using something other than a
     symbol in a place that a filehandle is expected. Here are
     ways to get indirect filehandles:

             $fh =   SOME_FH;       # bareword is strict-subs hostile
             $fh =  "SOME_FH";      # strict-refs hostile; same package only
             $fh =  *SOME_FH;       # typeglob
             $fh = \*SOME_FH;       # ref to typeglob (bless-able)
             $fh =  *SOME_FH{IO};   # blessed IO::Handle from *SOME_FH typeglob

     Or, you can use the "new" method from one of the IO::*
     modules to create an anonymous filehandle, store that in a
     scalar variable, and use it as though it were a normal

             use IO::Handle;                     # 5.004 or higher
             my $fh = IO::Handle->new();

     Then use any of those as you would a normal filehandle.
     Anywhere that Perl is expecting a filehandle, an indirect
     filehandle may be used instead. An indirect filehandle is

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     just a scalar variable that contains a filehandle. Functions
     like "print", "open", "seek", or the "<FH>" diamond operator
     will accept either a named filehandle or a scalar variable
     containing one:

             ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
             print $ofh "Type it: ";
             my $got = <$ifh>
             print $efh "What was that: $got";

     If you're passing a filehandle to a function, you can write
     the function in two ways:

             sub accept_fh {
                     my $fh = shift;
                     print $fh "Sending to indirect filehandle\n";

     Or it can localize a typeglob and use the filehandle

             sub accept_fh {
                     local *FH = shift;
                     print  FH "Sending to localized filehandle\n";

     Both styles work with either objects or typeglobs of real
     filehandles.  (They might also work with strings under some
     circumstances, but this is risky.)


     In the examples above, we assigned the filehandle to a
     scalar variable before using it. That is because only simple
     scalar variables, not expressions or subscripts of hashes or
     arrays, can be used with built-ins like "print", "printf",
     or the diamond operator. Using something other than a simple
     scalar variable as a filehandle is illegal and won't even

             my @fd = (*STDIN, *STDOUT, *STDERR);
             print $fd[1] "Type it: ";                           # WRONG
             my $got = <$fd[0]>                                  # WRONG
             print $fd[2] "What was that: $got";                 # WRONG

     With "print" and "printf", you get around this by using a
     block and an expression where you would place the

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             print  { $fd[1] } "funny stuff\n";
             printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
             # Pity the poor deadbeef.

     That block is a proper block like any other, so you can put
     more complicated code there. This sends the message out to
     one of two places:

             my $ok = -x "/bin/cat";
             print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
             print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";

     This approach of treating "print" and "printf" like object
     methods calls doesn't work for the diamond operator. That's
     because it's a real operator, not just a function with a
     comma-less argument. Assuming you've been storing typeglobs
     in your structure as we did above, you can use the built-in
     function named "readline" to read a record just as "<>"
     does. Given the initialization shown above for @fd, this
     would work, but only because readline() requires a typeglob.
     It doesn't work with objects or strings, which might be a
     bug we haven't fixed yet.

             $got = readline($fd[0]);

     Let it be noted that the flakiness of indirect filehandles
     is not related to whether they're strings, typeglobs,
     objects, or anything else.  It's the syntax of the
     fundamental operators. Playing the object game doesn't help
     you at all here.

  How can I set up a footer format to be used with write()?
     There's no builtin way to do this, but perlform has a couple
     of techniques to make it possible for the intrepid hacker.

  How can I write() into a string?
     See "Accessing Formatting Internals" in perlform for an
     "swrite()" function.

  How can I open a filehandle to a string?
     (contributed by Peter J. Holzer,

     Since Perl 5.8.0 a file handle referring to a string can be
     created by calling open with a reference to that string
     instead of the filename.  This file handle can then be used
     to read from or write to the string:

             open(my $fh, '>', \$string) or die "Could not open string for writing";
             print $fh "foo\n";
             print $fh "bar\n";      # $string now contains "foo\nbar\n"

             open(my $fh, '<', \$string) or die "Could not open string for reading";

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             my $x = <$fh>;  # $x now contains "foo\n"

     With older versions of Perl, the "IO::String" module
     provides similar functionality.

  How can I output my numbers with commas added?
     (contributed by brian d foy and Benjamin Goldberg)

     You can use Number::Format to separate places in a number.
     It handles locale information for those of you who want to
     insert full stops instead (or anything else that they want
     to use, really).

     This subroutine will add commas to your number:

             sub commify {
                     local $_  = shift;
                     1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
                     return $_;

     This regex from Benjamin Goldberg will add commas to


     It is easier to see with comments:

                     ^[-+]?             # beginning of number.
                     \d+?               # first digits before first comma
                     (?=                # followed by, (but not included in the match) :
                             (?>(?:\d{3})+) # some positive multiple of three digits.
                             (?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
                     |                  # or:
                     \G\d{3}            # after the last group, get three digits
                     (?=\d)             # but they have to have more digits after them.

  How can I translate tildes (~) in a filename?
     Use the <> ("glob()") operator, documented in perlfunc.
     Versions of Perl older than 5.6 require that you have a
     shell installed that groks tildes. Later versions of Perl
     have this feature built in. The "File::KGlob" module
     (available from CPAN) gives more portable glob

     Within Perl, you may use this directly:

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             $filename =~ s{
               ^ ~             # find a leading tilde
               (               # save this in $1
                   [^/]        # a non-slash character
                         *     # repeated 0 or more times (0 means me)
                   ? (getpwnam($1))[7]
                   : ( $ENV{HOME} || $ENV{LOGDIR} )

  How come when I open a file read-write it wipes it out?
     Because you're using something like this, which truncates
     the file and then gives you read-write access:

             open my $fh, '+>', '/path/name'; # WRONG (almost always)

     Whoops. You should instead use this, which will fail if the
     file doesn't exist.

             open my $fh, '+<', '/path/name'; # open for update

     Using ">" always clobbers or creates. Using "<" never does
     either. The "+" doesn't change this.

     Here are examples of many kinds of file opens. Those using
     sysopen() all assume

             use Fcntl;

     To open file for reading:

             open my $fh, '<', $path                                 or die $!;
             sysopen my $fh, $path, O_RDONLY                         or die $!;

     To open file for writing, create new file if needed or else
     truncate old file:

             open my $fh, '>', $path                                 or die $!;
             sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT         or die $!;
             sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666   or die $!;

     To open file for writing, create new file, file must not

             sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT          or die $!;
             sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666    or die $!;

     To open file for appending, create if necessary:

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             open my $fh, '>>' $path                                 or die $!;
             sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT        or die $!;
             sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666  or die $!;

     To open file for appending, file must exist:

             sysopen my $fh, $path, O_WRONLY|O_APPEND                or die $!;

     To open file for update, file must exist:

             open my $fh, '+<', $path                                or die $!;
             sysopen my $fh, $path, O_RDWR                           or die $!;

     To open file for update, create file if necessary:

             sysopen my $fh, $path, O_RDWR|O_CREAT                   or die $!;
             sysopen my $fh, $path, O_RDWR|O_CREAT, 0666             or die $!;

     To open file for update, file must not exist:

             sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT            or die $!;
             sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666      or die $!;

     To open a file without blocking, creating if necessary:

             sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT
                 or die "can't open /foo/somefile: $!":

     Be warned that neither creation nor deletion of files is
     guaranteed to be an atomic operation over NFS. That is, two
     processes might both successfully create or unlink the same
     file! Therefore O_EXCL isn't as exclusive as you might wish.

     See also the new perlopentut.

  Why do I sometimes get an "Argument list too long" when I use
     The "<>" operator performs a globbing operation (see above).
     In Perl versions earlier than v5.6.0, the internal glob()
     operator forks csh(1) to do the actual glob expansion, but
     csh can't handle more than 127 items and so gives the error
     message "Argument list too long". People who installed tcsh
     as csh won't have this problem, but their users may be
     surprised by it.

     To get around this, either upgrade to Perl v5.6.0 or later,
     do the glob yourself with readdir() and patterns, or use a
     module like File::KGlob, one that doesn't use the shell to
     do globbing.

  Is there a leak/bug in glob()?
     (contributed by brian d foy)

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     Starting with Perl 5.6.0, "glob" is implemented internally
     rather than relying on an external resource. As such, memory
     issues with "glob" aren't a problem in modern perls.

  How can I open a file with a leading ">" or trailing blanks?

     (contributed by Brian McCauley)

     The special two argument form of Perl's open() function
     ignores trailing blanks in filenames and infers the mode
     from certain leading characters (or a trailing "|"). In
     older versions of Perl this was the only version of open()
     and so it is prevalent in old code and books.

     Unless you have a particular reason to use the two argument
     form you should use the three argument form of open() which
     does not treat any characters in the filename as special.

             open my $fh, "<", "  file  ";  # filename is "   file   "
             open my $fh, ">", ">file";     # filename is ">file"

  How can I reliably rename a file?
     If your operating system supports a proper mv(1) utility or
     its functional equivalent, this works:

             rename($old, $new) or system("mv", $old, $new);

     It may be more portable to use the "File::Copy" module
     instead.  You just copy to the new file to the new name
     (checking return values), then delete the old one. This
     isn't really the same semantically as a "rename()", which
     preserves meta-information like permissions, timestamps,
     inode info, etc.

  How can I lock a file?
     Perl's builtin flock() function (see perlfunc for details)
     will call flock(2) if that exists, fcntl(2) if it doesn't
     (on perl version 5.004 and later), and lockf(3) if neither
     of the two previous system calls exists.  On some systems,
     it may even use a different form of native locking.  Here
     are some gotchas with Perl's flock():

     1.  Produces a fatal error if none of the three system calls
         (or their close equivalent) exists.

     2.  lockf(3) does not provide shared locking, and requires
         that the filehandle be open for writing (or appending,
         or read/writing).

     3.  Some versions of flock() can't lock files over a network
         (e.g. on NFS file systems), so you'd need to force the
         use of fcntl(2) when you build Perl.  But even this is

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         dubious at best. See the flock entry of perlfunc and the
         INSTALL file in the source distribution for information
         on building Perl to do this.

         Two potentially non-obvious but traditional flock
         semantics are that it waits indefinitely until the lock
         is granted, and that its locks are merely advisory. Such
         discretionary locks are more flexible, but offer fewer
         guarantees. This means that files locked with flock()
         may be modified by programs that do not also use
         flock(). Cars that stop for red lights get on well with
         each other, but not with cars that don't stop for red
         lights. See the perlport manpage, your port's specific
         documentation, or your system-specific local manpages
         for details. It's best to assume traditional behavior if
         you're writing portable programs.  (If you're not, you
         should as always feel perfectly free to write for your
         own system's idiosyncrasies (sometimes called
         "features").  Slavish adherence to portability concerns
         shouldn't get in the way of your getting your job done.)

         For more information on file locking, see also "File
         Locking" in perlopentut if you have it (new for 5.6).

  Why can't I just open(FH, ">file.lock")?
     A common bit of code NOT TO USE is this:

             sleep(3) while -e 'file.lock';  # PLEASE DO NOT USE
             open my $lock, '>', 'file.lock'; # THIS BROKEN CODE

     This is a classic race condition: you take two steps to do
     something which must be done in one. That's why computer
     hardware provides an atomic test-and-set instruction. In
     theory, this "ought" to work:

             sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
                     or die "can't open  file.lock: $!";

     except that lamentably, file creation (and deletion) is not
     atomic over NFS, so this won't work (at least, not every
     time) over the net.  Various schemes involving link() have
     been suggested, but these tend to involve busy-wait, which
     is also less than desirable.

  I still don't get locking. I just want to increment the number
     in the file. How can I do this?
     Didn't anyone ever tell you web-page hit counters were
     useless?  They don't count number of hits, they're a waste
     of time, and they serve only to stroke the writer's vanity.
     It's better to pick a random number; they're more realistic.

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     Anyway, this is what you can do if you can't help yourself.

             use Fcntl qw(:DEFAULT :flock);
             sysopen my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!";
             flock $fh, LOCK_EX                        or die "can't flock numfile: $!";
             my $num = <$fh> || 0;
             seek $fh, 0, 0                            or die "can't rewind numfile: $!";
             truncate $fh, 0                           or die "can't truncate numfile: $!";
             (print $fh $num+1, "\n")                  or die "can't write numfile: $!";
             close $fh                                 or die "can't close numfile: $!";

     Here's a much better web-page hit counter:

             $hits = int( (time() - 850_000_000) / rand(1_000) );

     If the count doesn't impress your friends, then the code
     might. :-)

  All I want to do is append a small amount of text to the end of
     a file. Do I still have to use locking?
     If you are on a system that correctly implements "flock" and
     you use the example appending code from "perldoc -f flock"
     everything will be OK even if the OS you are on doesn't
     implement append mode correctly (if such a system exists.)
     So if you are happy to restrict yourself to OSs that
     implement "flock" (and that's not really much of a
     restriction) then that is what you should do.

     If you know you are only going to use a system that does
     correctly implement appending (i.e. not Win32) then you can
     omit the "seek" from the code in the previous answer.

     If you know you are only writing code to run on an OS and
     filesystem that does implement append mode correctly (a
     local filesystem on a modern Unix for example), and you keep
     the file in block-buffered mode and you write less than one
     buffer-full of output between each manual flushing of the
     buffer then each bufferload is almost guaranteed to be
     written to the end of the file in one chunk without getting
     intermingled with anyone else's output. You can also use the
     "syswrite" function which is simply a wrapper around your
     system's write(2) system call.

     There is still a small theoretical chance that a signal will
     interrupt the system level "write()" operation before
     completion. There is also a possibility that some STDIO
     implementations may call multiple system level "write()"s
     even if the buffer was empty to start. There may be some
     systems where this probability is reduced to zero, and this
     is not a concern when using ":perlio" instead of your
     system's STDIO.

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  How do I randomly update a binary file?
     If you're just trying to patch a binary, in many cases
     something as simple as this works:

             perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

     However, if you have fixed sized records, then you might do
     something more like this:

             $RECSIZE = 220; # size of record, in bytes
             $recno   = 37;  # which record to update
             open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!";
             seek $fh, $recno * $RECSIZE, 0;
             read $fh, $record, $RECSIZE == $RECSIZE or die "can't read record $recno: $!";
             # munge the record
             seek $fh, -$RECSIZE, 1;
             print $fh $record;
             close $fh;

     Locking and error checking are left as an exercise for the
     reader.  Don't forget them or you'll be quite sorry.

  How do I get a file's timestamp in perl?
     If you want to retrieve the time at which the file was last
     read, written, or had its meta-data (owner, etc) changed,
     you use the -A, -M, or -C file test operations as documented
     in perlfunc.  These retrieve the age of the file (measured
     against the start-time of your program) in days as a
     floating point number. Some platforms may not have all of
     these times. See perlport for details. To retrieve the "raw"
     time in seconds since the epoch, you would call the stat
     function, then use "localtime()", "gmtime()", or
     "POSIX::strftime()" to convert this into human-readable

     Here's an example:

             my $write_secs = (stat($file))[9];
             printf "file %s updated at %s\n", $file,
             scalar localtime($write_secs);

     If you prefer something more legible, use the File::stat
     module (part of the standard distribution in version 5.004
     and later):

             # error checking left as an exercise for reader.
             use File::stat;
             use Time::localtime;
             my $date_string = ctime(stat($file)->mtime);
             print "file $file updated at $date_string\n";

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     The POSIX::strftime() approach has the benefit of being, in
     theory, independent of the current locale. See perllocale
     for details.

  How do I set a file's timestamp in perl?
     You use the utime() function documented in "utime" in
     perlfunc.  By way of example, here's a little program that
     copies the read and write times from its first argument to
     all the rest of them.

             if (@ARGV < 2) {
                     die "usage: cptimes timestamp_file other_files ...\n";
             my $timestamp = shift;
             my($atime, $mtime) = (stat($timestamp))[8,9];
             utime $atime, $mtime, @ARGV;

     Error checking is, as usual, left as an exercise for the

     The perldoc for utime also has an example that has the same
     effect as touch(1) on files that already exist.

     Certain file systems have a limited ability to store the
     times on a file at the expected level of precision. For
     example, the FAT and HPFS filesystem are unable to create
     dates on files with a finer granularity than two seconds.
     This is a limitation of the filesystems, not of utime().

  How do I print to more than one file at once?
     To connect one filehandle to several output filehandles, you
     can use the IO::Tee or Tie::FileHandle::Multiplex modules.

     If you only have to do this once, you can print individually
     to each filehandle.

             for my $fh (FH1, FH2, FH3) { print $fh "whatever\n" }

  How can I read in an entire file all at once?
     Are you sure you want to read the entire file and store it
     in memory?  If you mmap the file, you can virtually load the
     entire file into a string without actually storing it in

             use File::Map qw(map_file);

             map_file my $string, $filename;

     Once mapped, you can treat $string as you would any other
     string.  Since you don't actually load the data, mmap-ing is
     very fast and does not increase your memory footprint.

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     If you really want to load the entire file, you can use the
     "File::Slurp" module to do it in one step.

             use File::Slurp;

             my $all_of_it = read_file($filename); # entire file in scalar
             my @all_lines = read_file($filename); # one line per element

     The customary Perl approach for processing all the lines in
     a file is to do so one line at a time:

             open my $input, '<', $file or die "can't open $file: $!";
             while (<$input>) {
                     # do something with $_
             close $input or die "can't close $file: $!";

     This is tremendously more efficient than reading the entire
     file into memory as an array of lines and then processing it
     one element at a time, which is often--if not almost
     always--the wrong approach. Whenever you see someone do

             my @lines = <INPUT>;

     You should think long and hard about why you need everything
     loaded at once. It's just not a scalable solution. You might
     also find it more fun to use the standard Tie::File module,
     or the DB_File module's $DB_RECNO bindings, which allow you
     to tie an array to a file so that accessing an element the
     array actually accesses the corresponding line in the file.

     You can read the entire filehandle contents into a scalar.

             local $/;
             open my $fh, '<', $file or die "can't open $file: $!";
             $var = <$fh>;

     That temporarily undefs your record separator, and will
     automatically close the file at block exit. If the file is
     already open, just use this:

             $var = do { local $/; <$fh> };

     For ordinary files you can also use the read function.

             read( $fh, $var, -s $fh );

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     The third argument tests the byte size of the data on the
     INPUT filehandle and reads that many bytes into the buffer

  How can I read in a file by paragraphs?
     Use the $/ variable (see perlvar for details). You can
     either set it to "" to eliminate empty paragraphs
     ("abc\n\n\n\ndef", for instance, gets treated as two
     paragraphs and not three), or "\n\n" to accept empty

     Note that a blank line must have no blanks in it. Thus
     "fred\n \nstuff\n\n" is one paragraph, but
     "fred\n\nstuff\n\n" is two.

  How can I read a single character from a file?  From the
     You can use the builtin "getc()" function for most
     filehandles, but it won't (easily) work on a terminal
     device. For STDIN, either use the Term::ReadKey module from
     CPAN or use the sample code in "getc" in perlfunc.

     If your system supports the portable operating system
     programming interface (POSIX), you can use the following
     code, which you'll note turns off echo processing as well.

             #!/usr/bin/perl -w
             use strict;
             $| = 1;
             for (1..4) {
                     print "gimme: ";
                     my $got = getone();
                     print "--> $got\n";

             BEGIN {
             use POSIX qw(:termios_h);

             my ($term, $oterm, $echo, $noecho, $fd_stdin);

             my $fd_stdin = fileno(STDIN);

             $term     = POSIX::Termios->new();
             $oterm     = $term->getlflag();

             $echo     = ECHO | ECHOK | ICANON;
             $noecho   = $oterm & ~$echo;

             sub cbreak {

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                     $term->setcc(VTIME, 1);
                     $term->setattr($fd_stdin, TCSANOW);

             sub cooked {
                     $term->setcc(VTIME, 0);
                     $term->setattr($fd_stdin, TCSANOW);

             sub getone {
                     my $key = '';
                     sysread(STDIN, $key, 1);
                     return $key;


             END { cooked() }

     The Term::ReadKey module from CPAN may be easier to use.
     Recent versions include also support for non-portable
     systems as well.

             use Term::ReadKey;
             open my $tty, '<', '/dev/tty';
             print "Gimme a char: ";
             ReadMode "raw";
             my $key = ReadKey 0, $tty;
             ReadMode "normal";
             printf "\nYou said %s, char number %03d\n",
                     $key, ord $key;

  How can I tell whether there's a character waiting on a
     The very first thing you should do is look into getting the
     Term::ReadKey extension from CPAN. As we mentioned earlier,
     it now even has limited support for non-portable (read: not
     open systems, closed, proprietary, not POSIX, not Unix,
     etc.) systems.

     You should also check out the Frequently Asked Questions
     list in comp.unix.* for things like this: the answer is
     essentially the same.  It's very system dependent. Here's
     one solution that works on BSD systems:

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             sub key_ready {
                     my($rin, $nfd);
                     vec($rin, fileno(STDIN), 1) = 1;
                     return $nfd = select($rin,undef,undef,0);

     If you want to find out how many characters are waiting,
     there's also the FIONREAD ioctl call to be looked at. The
     h2ph tool that comes with Perl tries to convert C include
     files to Perl code, which can be "require"d. FIONREAD ends
     up defined as a function in the sys/ file:

             require 'sys/';

             $size = pack("L", 0);
             ioctl(FH, FIONREAD(), $size)    or die "Couldn't call ioctl: $!\n";
             $size = unpack("L", $size);

     If h2ph wasn't installed or doesn't work for you, you can
     grep the include files by hand:

             % grep FIONREAD /usr/include/*/*
             /usr/include/asm/ioctls.h:#define FIONREAD      0x541B

     Or write a small C program using the editor of champions:

             % cat > fionread.c
             #include <sys/ioctl.h>
             main() {
                 printf("%#08x\n", FIONREAD);
             % cc -o fionread fionread.c
             % ./fionread

     And then hard code it, leaving porting as an exercise to
     your successor.

             $FIONREAD = 0x4004667f;         # XXX: opsys dependent

             $size = pack("L", 0);
             ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
             $size = unpack("L", $size);

     FIONREAD requires a filehandle connected to a stream,
     meaning that sockets, pipes, and tty devices work, but not

  How do I do a "tail -f" in perl?
     First try

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             seek(GWFILE, 0, 1);

     The statement "seek(GWFILE, 0, 1)" doesn't change the
     current position, but it does clear the end-of-file
     condition on the handle, so that the next "<GWFILE>" makes
     Perl try again to read something.

     If that doesn't work (it relies on features of your stdio
     implementation), then you need something more like this:

             for (;;) {
               for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
                 # search for some stuff and put it into files
               # sleep for a while
               seek(GWFILE, $curpos, 0);  # seek to where we had been

     If this still doesn't work, look into the "clearerr" method
     from "IO::Handle", which resets the error and end-of-file
     states on the handle.

     There's also a "File::Tail" module from CPAN.

  How do I dup() a filehandle in Perl?
     If you check "open" in perlfunc, you'll see that several of
     the ways to call open() should do the trick. For example:

             open my $log, '>>', '/foo/logfile';
             open STDERR, '>&LOG';

     Or even with a literal numeric descriptor:

             my $fd = $ENV{MHCONTEXTFD};
             open $mhcontext, "<&=$fd";  # like fdopen(3S)

     Note that "<&STDIN" makes a copy, but "<&=STDIN" make an
     alias. That means if you close an aliased handle, all
     aliases become inaccessible. This is not true with a copied

     Error checking, as always, has been left as an exercise for
     the reader.

  How do I close a file descriptor by number?
     If, for some reason, you have a file descriptor instead of a
     filehandle (perhaps you used "POSIX::open"), you can use the
     "close()" function from the "POSIX" module:

             use POSIX ();

             POSIX::close( $fd );

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     This should rarely be necessary, as the Perl "close()"
     function is to be used for things that Perl opened itself,
     even if it was a dup of a numeric descriptor as with
     "MHCONTEXT" above. But if you really have to, you may be
     able to do this:

             require 'sys/';
             my $rc = syscall(&SYS_close, $fd + 0);  # must force numeric
             die "can't sysclose $fd: $!" unless $rc == -1;

     Or, just use the fdopen(3S) feature of "open()":

             open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
             close $fh;

  Why can't I use "C:\temp\foo" in DOS paths?  Why doesn't
     `C:\temp\foo.exe` work?
     Whoops!  You just put a tab and a formfeed into that
     filename!  Remember that within double quoted strings
     ("like\this"), the backslash is an escape character. The
     full list of these is in "Quote and Quote-like Operators" in
     perlop. Unsurprisingly, you don't have a file called
     "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on
     your legacy DOS filesystem.

     Either single-quote your strings, or (preferably) use
     forward slashes.  Since all DOS and Windows versions since
     something like MS-DOS 2.0 or so have treated "/" and "\" the
     same in a path, you might as well use the one that doesn't
     clash with Perl--or the POSIX shell, ANSI C and C++, awk,
     Tcl, Java, or Python, just to mention a few. POSIX paths are
     more portable, too.

  Why doesn't glob("*.*") get all the files?
     Because even on non-Unix ports, Perl's glob function follows
     standard Unix globbing semantics. You'll need "glob("*")" to
     get all (non-hidden) files. This makes glob() portable even
     to legacy systems. Your port may include proprietary
     globbing functions as well. Check its documentation for

  Why does Perl let me delete read-only files?  Why does "-i"
     clobber protected files?  Isn't this a bug in Perl?
     This is elaborately and painstakingly described in the file-
     dir-perms article in the "Far More Than You Ever Wanted To
     Know" collection in .

     The executive summary: learn how your filesystem works. The
     permissions on a file say what can happen to the data in

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     that file.  The permissions on a directory say what can
     happen to the list of files in that directory. If you delete
     a file, you're removing its name from the directory (so the
     operation depends on the permissions of the directory, not
     of the file). If you try to write to the file, the
     permissions of the file govern whether you're allowed to.

  How do I select a random line from a file?
     Short of loading the file into a database or pre-indexing
     the lines in the file, there are a couple of things that you
     can do.

     Here's a reservoir-sampling algorithm from the Camel Book:

             rand($.) < 1 && ($line = $_) while <>;

     This has a significant advantage in space over reading the
     whole file in. You can find a proof of this method in The
     Art of Computer Programming, Volume 2, Section 3.4.2, by
     Donald E. Knuth.

     You can use the "File::Random" module which provides a
     function for that algorithm:

             use File::Random qw/random_line/;
             my $line = random_line($filename);

     Another way is to use the "Tie::File" module, which treats
     the entire file as an array. Simply access a random array

  Why do I get weird spaces when I print an array of lines?
     (contributed by brian d foy)

     If you are seeing spaces between the elements of your array
     when you print the array, you are probably interpolating the
     array in double quotes:

             my @animals = qw(camel llama alpaca vicuna);
             print "animals are: @animals\n";

     It's the double quotes, not the "print", doing this.
     Whenever you interpolate an array in a double quote context,
     Perl joins the elements with spaces (or whatever is in $",
     which is a space by default):

             animals are: camel llama alpaca vicuna

     This is different than printing the array without the

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             my @animals = qw(camel llama alpaca vicuna);
             print "animals are: ", @animals, "\n";

     Now the output doesn't have the spaces between the elements
     because the elements of @animals simply become part of the
     list to "print":

             animals are: camelllamaalpacavicuna

     You might notice this when each of the elements of @array
     end with a newline. You expect to print one element per
     line, but notice that every line after the first is

             this is a line
              this is another line
              this is the third line

     That extra space comes from the interpolation of the array.
     If you don't want to put anything between your array
     elements, don't use the array in double quotes. You can send
     it to print without them:

             print @lines;

  How do I traverse a directory tree?
     (contributed by brian d foy)

     The "File::Find" module, which comes with Perl, does all of
     the hard work to traverse a directory structure. It comes
     with Perl. You simply call the "find" subroutine with a
     callback subroutine and the directories you want to

             use File::Find;

             find( \&wanted, @directories );

             sub wanted {
                     # full path in $File::Find::name
                     # just filename in $_
                     ... do whatever you want to do ...

     The "File::Find::Closures", which you can download from
     CPAN, provides many ready-to-use subroutines that you can
     use with "File::Find".

     The "File::Finder", which you can download from CPAN, can
     help you create the callback subroutine using something
     closer to the syntax of the "find" command-line utility:

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             use File::Find;
             use File::Finder;

             my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');

             find( $deep_dirs->as_options, @places );

     The "File::Find::Rule" module, which you can download from
     CPAN, has a similar interface, but does the traversal for
     you too:

             use File::Find::Rule;

             my @files = File::Find::Rule->file()
                                                              ->name( '*.pm' )
                                                              ->in( @INC );

  How do I delete a directory tree?
     (contributed by brian d foy)

     If you have an empty directory, you can use Perl's built-in
     "rmdir".  If the directory is not empty (so, no files or
     subdirectories), you either have to empty it yourself (a lot
     of work) or use a module to help you.

     The "File::Path" module, which comes with Perl, has a
     "remove_tree" which can take care of all of the hard work
     for you:

             use File::Path qw(remove_tree);

             remove_tree( @directories );

     The "File::Path" module also has a legacy interface to the
     older "rmtree" subroutine.

  How do I copy an entire directory?
     (contributed by Shlomi Fish)

     To do the equivalent of "cp -R" (i.e. copy an entire
     directory tree recursively) in portable Perl, you'll either
     need to write something yourself or find a good CPAN module
     such as  File::Copy::Recursive.

     Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington,
     and other authors as noted. All rights reserved.

     This documentation is free; you can redistribute it and/or
     modify it under the same terms as Perl itself.

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     Irrespective of its distribution, all code examples here are
     in the public domain. You are permitted and encouraged to
     use this code and any derivatives thereof in your own
     programs for fun or for profit as you see fit. A simple
     comment in the code giving credit to the FAQ would be
     courteous but is not required.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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