Restructure the heredocs in perlmodlib.PL so the file as-is is valid Pod.

Restructure the heredocs in perlmodlib.PL so the file as-is is valid Pod.

Using =cut as the heredoc terminator makes the whole file parse as
(reasonably) sane Pod as-is to anything that attempts to brute-force treat
it as such. The content is already useful - this just makes it tidier, by
stopping anything doing this mistaking the rest of the Perl code for Pod.