Actually I think += and especially -= should be modified. None of them makes much sense when used on a string, so they should behave this way:

      my $string= "mirod";
      $string += " is dumb"; # $string is now "mirod is dumb"
      $string -= "Sure ";    # $string is now "Sure mirod is dumb"
[download]

Although I know this would start a religious war with those who want -= to behave like this:

      $string="I am _not_ dumb";
      $string -= "_not_ ";        # now guess what's in $string?
[download]

I am ready to compromise and go for the second version only if >> and << are re-used too (after all who uses them on strings?) for my original proposal:

      my $string= "can";
      $string >> " compromise"; # $string is now "can compromise"
      $string << "I ";          # $string is now "I can compromise"
[download]

As for <= of course it should do a deep copy of a reference while for consistency <- would do a shallow copy (just like = does).