http://www.perlmonks.org?node_id=924909


in reply to Perl6: What is "idfirst letter"?

“idfirst” is jut an abbreviation for any code point with the Unicode ID_Start property.

Anyway, Perl identifiers don’t seem to pay as much attention to IDS as you would think. You can start them with other types of decimal numbers and connector punctuation, both of which are IDC (ID_Continue) not IDS.