#!/usr/bin/perl -w

my @strings = qw( google mississippi that people );
my $alphabet = join "", ( 'a'..'z' );

foreach ( @strings ) {
    print c( $_, $alphabet ), "\n";
}

sub c {
    my ( $string, $alpha ) = @_;
    my @letters = split //, $string;
    my @alphabet = split //, $alpha;
    my %hash;
    my @result;
    my $i = 0;
    foreach ( @letters ) {
        if (!defined( $hash{ $_ } )) {
            $hash{ $_ } = $alphabet[ $i++ ];
        }
        push @result, $hash{ $_ };
    }        
    return join "", @result;
}

# Prints out:
#abbacd
#abccbccbddb
#abca
#abcadb