>perl588 -le"$re=qr/a$/; print $re; print qq{a\nb\n}=~/$re/m ?1:0"
(?-xism:foo$)
1

>perl5100 -le"$re=qr/a$/; print $re; print qq{a\nb\n}=~/$re/m ?1:0"
(?-xism:foo$)
0