In general, the 5.6.1 version of Perl is compatible with the previous version. Scripts do not have to be rebuilt or recompiled to function. However, any XSUB-based (.xs) modules require recompilation and reinstallation.
In the Solaris 9 release, you can access the older version of Perl as /usr/perl5/5.00503/bin/perl. The older version might not be supported in future releases and this version should only be used until the new modules are rebuilt.