Also remember that m// behaves differently depending on context, flags, whether () groups are present...

In particular, when $str =~ m//g is used in scalar context (as in your posted examples), it requests match position pos($str) to be remembered; further m//g continue from there. (But assigning to $str will forget the pos!)

  # m// in list context, no g flag
  if (my @match = $str =~ m/(pattern1)(pattern2)/i) {
    $str = function($match[0]);
    $var1 = function($match[1]);
  }
[download]