## initialization
my $old = 'abc';
my $new = 'xyz';
my %table; @table{split //, $old} = split //, $new;
my $table_re = join '|', map quotemeta reverse sort keys %table;

## the deed
my $string = 'abracadabra';
$string =~ s/($table_re)/$table{$1}/g;