#!/usr/local/bin/perl -l -w

use strict;

my $str = "SAdBoy";

$str =~ s/($str)/fix_case($1, 'badgirl')/eig;

print $str;

sub fix_case {
 my ($match_word, $replace_word) = @_;
 my @rep = split //, $replace_word;
 my $i = 0;
 for (split '', $match_word) {
  $rep[$i] = $_ eq lc($_) ? lc($rep[$i++]) : uc($rep[$i++]);
 }
 return join '', @rep;
}