my %hash = ('Abend' => 'NN',
            'Sonne' => 'NN',
            'Haus' => 'NN',
            'Abendsonne' => 'who knows',
);

for my $word ( keys %hash ) {
    for my $pre ( keys %hash ) {
        for my $post ( keys %hash ) {
            if ( "\L$word" eq "\L$pre$post" ) {
                print $word, $/;
            }
        }
    }
}