s/(.)/$hash{$1}/eg
[download]

Will end up removing all chars in the string that are not specifically listed in the hash.

I also believe that tr/// is always faster than s///.

#!/usr/local/bin/perl
use strict;

use Benchmark;

timethese( 1000000, {
    trchange    => \&trchange,
    schange => \&schange,
});

sub trchange
{
    my $s = q{123 abc def a b a b a b};
    $s =~ tr/a/A/;
}

sub schange
{
    my $s = q{123 abc def a b a b a b};
    $s =~ s/a/A/g;
}

# Benchmark: timing 1000000 iterations of schange, trchange...
#   schange: 21 wallclock secs (21.39 usr +  0.01 sys = 21.40 CPU) @ 4
+6728.97/s (n=1000000)
#  trchange:  7 wallclock secs ( 6.41 usr +  0.01 sys =  6.42 CPU) @ 1
+55763.24/s (n=1000000)
[download]

Wonko