These match:

# (not using 'say' because I wanted to try it on old perl versions)
perl -e 'printf "%s\n", "ab\nab\n" =~ /^ab$ab$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$ab$)/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$ab)$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab)$ab$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(a)b$ab$/m'
[download]

(with '$' consuming a "\n" because it is /m mode)

These do not match:

perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$a)b$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$)ab$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^ab$(ab)$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$){2}/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /(ab$){2}/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /(?^:^(?^m:(ab$){2}))/m'
[download]

This is greedy but only matches one line

perl -e 'printf "%s\n", "ab\nab\n" =~ /^((?:ab$)+)/m'
[download]

and it seems to me that all of them should match. This appears to happen as old as 5.8.9 and as new as 5.32.1

Any insights?