Printing the pre-compiled regex, I get:

 3:05 >perl -wE "my $q = qr/^ab$/; say $q;"
(?^u:^ab$)

 3:06 >
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and from perlre#Extended-Patterns it appears that this is equivalent to (?ud-imsx:^ab$), which explicitly turns off the m modifier throughout of the embeded pattern-match modifier. So adding /m to the regex has no effect, because it is overridden within the (?-m:...) construct inside the regex.

I don’t know how all this worked in earlier versions of Perl, but in recent versions, it seems you have to add the /m modifier when the regex is first constructed using qr//:

my $qre = qr/^ab$/m;
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Hope that helps,