$p = '4+5*6';   $re = qr/$p/;  $str =~ $re;      # Still doesn't.
$p = 'm/a/';    $re = qr/$p/;  $str =~ $re;      # Still doesn't.
$p = 's/a/b/';  $re = qr/$p/;  $str =~ $re;      # Still doesn't.