#!/usr/bin/perl
use strict;
use warnings;
use String::Combination qw(combination);

open DICT, 'english-words.txt' or die "Can't open dictionary: $!";
my @dict = <DICT>;
chomp @dict;
my %dict = map { (lc($_) => 1) } @dict;

print "Letters? ";
my $letters = lc <STDIN>;
chomp $letters;

my @results;
for (my $i = 2; $i <= length $letters; $i++) {
    push @results, grep { $dict{$_} } combination $letters, $i;
}

print "@results";
__END__
Output:
Letters? tar
at art rat tar