includes

Since you use this term, just want to make certain that you're checking both for use and require and that you know that these can appear practically anywhere in the file.

If you find no other sources, maybe rest simply isn't defined at all. For example:
perl -e 'if(0) { $return_line = rest($short_module, $peakapex); } else { print "foo\n"; }'
will run without error (ignoring strict and warnings for the moment). Probably not so obvious in your program but you can check for this situation with a nearby print statement or, of course, a proper debugger.

If even this doesn't explain it, the definition of rest must be hidden in you file somehow.