Just a guess, but the delta for 5.20 includes this entry:

Executing a regex that contains the ^ anchor (or its variant under the /m flag) has been made much faster in several situations.

https://metacpan.org/pod/release/RJBS/perl-5.20.0/pod/perldelta.pod#Performance-Enhancements

Maybe that enhancement has some side effects triggered by the \s* ^ \s* patterns in the regexps.

Are you able to test what happens in pre-5.20 if these patterns are changed to \s*?

Edit - I should have asked for what happens either side of 5.20.