Do sources still exist for Perl 4 and earlier?
I decided to go looking for when Perl got verbose regex. The oldest thing I can find on perldoc.perl.org or cpan.org is 5.004 (1997), where they were an existing feature.
EDIT: Found 4.036 sources (1993). A quick scan of the man page (troff source!) does not find verbose regular expressions. So it looks like they were introduced very early in the Perl 5 series.