You can download Perl 5.8.7 for the NSK OSS environment from the ITUGLIB Open Source Technical Library. I've used it on OSS (a Unix-like environment) and it works reasonably well. You can interoperate with the Guardian environment from OSS by running the gtacl command via Perl's system function or backticks. IIRC, I had to make a one-line change to the File::Find module (changed the value of one of the link variables I think, update: added the line $File::Find::dont_use_nlink = 1; to Find.pm) to get it to work reliably. And setting up the networking (I copied /etc/hosts, /etc/services, /etc/protocols, /etc/inetd.conf and a bunch of other Unix-like files across from a Unix box) was a pest.

Last time I looked (around five years ago), building Perl from source on OSS was not trivial. The ITUGLIB Perl was built using a "HP Internal" version of gcc that was not generally available. Update: So installing CPAN modules with C/xs components is problematic. Pure Perl CPAN modules are fine though, and I successfully installed a number of those.

A very old version of Perl (5.004 I think) is available for the Guardian environment, though I've not used it. Guardian is not at all Unix-like (e.g. file names limited to eight characters in length with no file extension and lacking Unix-like hierarchical directories), so this is an ancient custom fork and a lot of Unix-like Perl features will necessarily be missing on Guardian.