This greatly depends on what they compiled it with. Was it one of Perl's own compiler backends? If so, was it B::C or B::CC? Did they use something else like PerlApp or Perl2Exe?

I believe the latter two embed the source in the executable more or less verbatim. These would be relatively easy to reverse.

A B::C binary contains a prebuilt optree that is run using an embedded interpreter; this might be possible to salvage with some debugger wizardry and B::Deparse.

If they used B::CC you're probably out of luck.