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perlstyle (1)


perlstyle - Perl style guide


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Perl Programmers Reference Guide                     PERLSTYLE(1)

     perlstyle - Perl style guide

     Each programmer will, of course, have his or her own
     preferences in regards to formatting, but there are some
     general guidelines that will make your programs easier to
     read, understand, and maintain.

     The most important thing is to run your programs under the
     -w flag at all times.  You may turn it off explicitly for
     particular portions of code via the "no warnings" pragma or
     the $^W variable if you must.  You should also always run
     under "use strict" or know the reason why not.  The "use
     sigtrap" and even "use diagnostics" pragmas may also prove

     Regarding aesthetics of code lay out, about the only thing
     Larry cares strongly about is that the closing curly bracket
     of a multi-line BLOCK should line up with the keyword that
     started the construct.  Beyond that, he has other
     preferences that aren't so strong:

     o   4-column indent.

     o   Opening curly on same line as keyword, if possible,
         otherwise line up.

     o   Space before the opening curly of a multi-line BLOCK.

     o   One-line BLOCK may be put on one line, including

     o   No space before the semicolon.

     o   Semicolon omitted in "short" one-line BLOCK.

     o   Space around most operators.

     o   Space around a "complex" subscript (inside brackets).

     o   Blank lines between chunks that do different things.

     o   Uncuddled elses.

     o   No space between function name and its opening

     o   Space after each comma.

     o   Long lines broken after an operator (except "and" and

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     o   Space after last parenthesis matching on current line.

     o   Line up corresponding items vertically.

     o   Omit redundant punctuation as long as clarity doesn't

     Larry has his reasons for each of these things, but he
     doesn't claim that everyone else's mind works the same as
     his does.

     Here are some other more substantive style issues to think

     o   Just because you CAN do something a particular way
         doesn't mean that you SHOULD do it that way.  Perl is
         designed to give you several ways to do anything, so
         consider picking the most readable one.  For instance

             open(FOO,$foo) || die "Can't open $foo: $!";

         is better than

             die "Can't open $foo: $!" unless open(FOO,$foo);

         because the second way hides the main point of the
         statement in a modifier.  On the other hand

             print "Starting analysis\n" if $verbose;

         is better than

             $verbose && print "Starting analysis\n";

         because the main point isn't whether the user typed -v
         or not.

         Similarly, just because an operator lets you assume
         default arguments doesn't mean that you have to make use
         of the defaults.  The defaults are there for lazy
         systems programmers writing one-shot programs.  If you
         want your program to be readable, consider supplying the

         Along the same lines, just because you CAN omit
         parentheses in many places doesn't mean that you ought

             return print reverse sort num values %array;
             return print(reverse(sort num (values(%array))));

         When in doubt, parenthesize.  At the very least it will

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         let some poor schmuck bounce on the % key in vi.

         Even if you aren't in doubt, consider the mental welfare
         of the person who has to maintain the code after you,
         and who will probably put parentheses in the wrong

     o   Don't go through silly contortions to exit a loop at the
         top or the bottom, when Perl provides the "last"
         operator so you can exit in the middle.  Just "outdent"
         it a little to make it more visible:

                 for (;;) {
                   last LINE if $foo;
                     next LINE if /^#/;

     o   Don't be afraid to use loop labels--they're there to
         enhance readability as well as to allow multilevel loop
         breaks.  See the previous example.

     o   Avoid using "grep()" (or "map()") or `backticks` in a
         void context, that is, when you just throw away their
         return values.  Those functions all have return values,
         so use them.  Otherwise use a "foreach()" loop or the
         "system()" function instead.

     o   For portability, when using features that may not be
         implemented on every machine, test the construct in an
         eval to see if it fails.  If you know what version or
         patchlevel a particular feature was implemented, you can
         test $] ($PERL_VERSION in "English") to see if it will
         be there.  The "Config" module will also let you
         interrogate values determined by the Configure program
         when Perl was installed.

     o   Choose mnemonic identifiers.  If you can't remember what
         mnemonic means, you've got a problem.

     o   While short identifiers like $gotit are probably ok, use
         underscores to separate words in longer identifiers.  It
         is generally easier to read $var_names_like_this than
         $VarNamesLikeThis, especially for non-native speakers of
         English. It's also a simple rule that works consistently
         with "VAR_NAMES_LIKE_THIS".

         Package names are sometimes an exception to this rule.
         Perl informally reserves lowercase module names for
         "pragma" modules like "integer" and "strict".  Other

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         modules should begin with a capital letter and use mixed
         case, but probably without underscores due to
         limitations in primitive file systems' representations
         of module names as files that must fit into a few sparse

     o   You may find it helpful to use letter case to indicate
         the scope or nature of a variable. For example:

             $ALL_CAPS_HERE   constants only (beware clashes with perl vars!)
             $Some_Caps_Here  package-wide global/static
             $no_caps_here    function scope my() or local() variables

         Function and method names seem to work best as all
         lowercase.  E.g., "$obj->as_string()".

         You can use a leading underscore to indicate that a
         variable or function should not be used outside the
         package that defined it.

     o   If you have a really hairy regular expression, use the
         "/x" modifier and put in some whitespace to make it look
         a little less like line noise.  Don't use slash as a
         delimiter when your regexp has slashes or backslashes.

     o   Use the new "and" and "or" operators to avoid having to
         parenthesize list operators so much, and to reduce the
         incidence of punctuation operators like "&&" and "||".
         Call your subroutines as if they were functions or list
         operators to avoid excessive ampersands and parentheses.

     o   Use here documents instead of repeated "print()"

     o   Line up corresponding things vertically, especially if
         it'd be too long to fit on one line anyway.

             $IDX = $ST_MTIME;
             $IDX = $ST_ATIME       if $opt_u;
             $IDX = $ST_CTIME       if $opt_c;
             $IDX = $ST_SIZE        if $opt_s;

             mkdir $tmpdir, 0700 or die "can't mkdir $tmpdir: $!";
             chdir($tmpdir)      or die "can't chdir $tmpdir: $!";
             mkdir 'tmp',   0777 or die "can't mkdir $tmpdir/tmp: $!";

     o   Always check the return codes of system calls.  Good
         error messages should go to "STDERR", include which
         program caused the problem, what the failed system call
         and arguments were, and (VERY IMPORTANT) should contain
         the standard system error message for what went wrong.
         Here's a simple but sufficient example:

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             opendir(D, $dir)     or die "can't opendir $dir: $!";

     o   Line up your transliterations when it makes sense:

             tr [abc]

     o   Think about reusability.  Why waste brainpower on a one-
         shot when you might want to do something like it again?
         Consider generalizing your code.  Consider writing a
         module or object class.  Consider making your code run
         cleanly with "use strict" and "use warnings" (or -w) in
         effect.  Consider giving away your code.  Consider
         changing your whole world view.  Consider... oh, never

     o   Try to document your code and use Pod formatting in a
         consistent way. Here are commonly expected conventions:

         o   use "C<>" for function, variable and module names
             (and more generally anything that can be considered
             part of code, like filehandles or specific values).
             Note that function names are considered more
             readable with parentheses after their name, that is

         o   use "B<>" for commands names like cat or grep.

         o   use "F<>" or "C<>" for file names. "F<>" should be
             the only Pod code for file names, but as most Pod
             formatters render it as italic, Unix and Windows
             paths with their slashes and backslashes may be less
             readable, and better rendered with "C<>".

     o   Be consistent.

     o   Be nice.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |

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     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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