my $string = 'fred=abcdef&wilbur=123456&fred=hijk';

##</code><code>##

sub obscure {
   my ($self, $s) = @_;
   use constant REPLACEMENT => '*';

   my @tags = qw/fred wilbur/;

   for my $tag (@tags) {
       $s =~ s{$tag=(.*?)\&}{
          "$tag=" . (REPLACEMENT x length($1)) . '&'
       }gex;
       $s =~ s/$tag=(.*)$/"$tag="/gex;  # WRONG
   }

   return $s;
}