The difference is in (?-xism: vs (?^: which is what I get.

It means the same thing, see http://perldoc.perl.org/5.12.0/perlre.html#(?pimsx-imsx), http://perldoc.perl.org/5.14.0/perlre.html#(?^alupimsx)

$ perl5.12.2.exe -e " print qr// "
(?-xism:)

$ perl5.14.1.exe -e " print qr// "
(?^:)
[download]