# takes an integer word length and a list of letters,
# returns a list of words of the integer length
#
{ my %cache;
  sub wordz {
    my ( $n, @letters ) = @_;
    my $cache_key = join( '', @letters, $n );

    return if $n > @letters;
    return @letters if $n == 1;

    return @{ $cache{ $cache_key } } if exists $cache{ $cache_key };

    my @wordz;
    for ( my $i = 0; $i <= $#letters; $i++ ) {

      push @wordz, map { $letters[ $i ] . $_ }
        wordz( $n - 1, @letters[ 0 .. $i - 1 ],
                       @letters[ $i + 1 .. $#letters ] );

      my @o = wordz( $n, @letters[ $i + 1 .. $#letters ] );

      push @wordz, @o;
    }
    $cache{ $cache_key } = \@wordz;

    return @wordz;
  }
}
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It is also fun to do this problem non-recursively. If you are interested in that, I also have code (though there are different possible approaches).