I haven't grokked your code, but I'd suggest it's as simple as this:

$string = join '', 'a'..'z';
@find = 'a'..'g';
@change = 'h'..'m';
foreach $i (0..$#find)
{
  $string =~ s/$find[$i]/$change[$i]/g;
}
[download]

Update: left off the /g