env(perl) Much improved regex detection

[joshgoebel]

Resolves #2952.

Changes

Added many more common regex variants to perl.

This does not attempt to solve:

say ("fee" =~ s aeaoagr . "bar");

Regex with entirely arbitrary characters are a pain because they
could potentialy match so many languages, and slow down auto-detect
so we will try to avoid them for now.

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md
• Added myself to AUTHORS.txt, under Contributors