I'd like comments on this bit of Perl. Any comments at all. Good, bad... whatever. Please? :-)

sub ModifyUrl {
  my ($keytoreplace, $keyvalue, $newurl) = (@_[0], @_[1], @_[2]);
  if (!$newurl) { $newurl = $ENV{'REQUEST_URI'}; }
  $newurl =~ s/(\?|&)$keytoreplace=[^&]+//;
  if ($keyvalue) {
    if ($newurl =~ m/(\?|&)/) { $newurl = $newurl."&$keytoreplace=$keyvalue"; } else {
    $newurl = $newurl."?$keytoreplace=$keyvalue"; }
  }
  if ($newurl !~ m/\?/) { $newurl =~ s/&/\?/; }
  return $newurl;
}