#! /usr/bin/perl -w
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)],
);

foreach my $comb(2222..9999)
{
    combs('', split //, $comb) unless $comb =~ /[10]/;
}

sub combs {
    my $word = shift;
    my $first = shift;
    foreach my $combination (@{$digit_letters{$first}}) {
        if (@_) {
            combs($word.$combination, @_);
        }
        else {
            print "$word$combination\n";
        }
    }
}
[download]