How on earth could anyone or anything figure out what is going on in the regex that lies in the middle of otherwise short module ...

The docs say the regexp is taken from Email::Address, which makes it a lot clearer how it is put together: I guess you'd start at line 137: our $addr_spec  = qr/$local_part\@$domain/; and work backwards from there.

We also read there that this is implementing RFC 2822, which mandates what a valid email address consists of. At 51 pages - that describe the whole message format, not just email addresses - that's pretty light as standards documents go. :)