[Solved] how to pass hash reference to a subroutine

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You never assigned to $hash_1 and $hash_2!

my ($hash_1, $hash_2) = @_;

You have more problems than that, however, both keys and values in scalar context return the number of elements in a hash, so you are simply checking if both hashes are of the same size!

sub are_hashes_equal {
    my ($hash1, $hash2) = @_;

    {
        my %set;
        ++$set{$_} for keys(%$hash1);
        --$set{$_} for keys(%$hash2);
        return 0 if grep { $_ } values(%set);
    }

    for my $key (keys(%$hash1)) {
        return 0 if $hash1->{$key} ne $hash2->{$key};
    }

    return 1;
}

solved how to pass hash reference to a subroutine