use Data::Dumper;

my %hash = (
    Titanic => [ "Anno 1912", "Southampton" ],
    Cheopspyramide => [ "Anno ca. -3000 plusminus 1500", "Gizeh" ],
    Perl => [ "Kamel" ],
);

my %reverse_lookup;

foreach my $arrayref ( keys %hash ) {
    foreach my $element ( @{ $hash{$arrayref} } ) {
        $reverse_lookup{$element} = $arrayref;
    }
}

print Dumper \%reverse_lookup;