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

my %digit_letters = (
    2 => [qw(a b c)],
    3 => [qw(d e f)],
    4 => [qw(g h i)],
    5 => [qw(j k l)],
    6 => [qw(m n o)],
    7 => [qw(p r s)],
    8 => [qw(t u v)],
    9 => [qw(w x y)],
);

my @letter_combinations;

my $letterchooser = choose([0 .. 2], 4);
while (my $letters = $letterchooser->()) {
    push @letter_combinations, $letters;
}

my $digitchooser = choose([2 .. 9], 4);
while (my $digits = $digitchooser->()) {
    for my $letters (@letter_combinations) {
        my @digits  = split //, $digits;
        my @letters = split //, $letters;

        my @word = map { $digit_letters{$digits[$_]}[$letters[$_]] }
                   0 .. $#digits;

        print "$digits: ", @word, "\n";
    }
}

sub basen {
    my ($base, $num) = @_;

    my $q = int($num / $base);
    my $r =     $num % $base;

    return $r if $q == 0;
    return basen($base, $q), $r;
}

sub choose {
    my ($list, $number) = @_;

    my $listcount = @$list;
    my $combcount = @$list**$number;

    my $curr = 0;

    sub {
        return if $curr >= $combcount;

        my @choice = basen($listcount, $curr++);
        unshift @choice, 0 while @choice < $number;

        return join "", map $list->[$_], @choice;
    }
}