I'd solve it this way:

sub two_sum_choroba {
    my ($input, $target) = @_;
    my %seen;
    for my $i (0 .. $#$input) {
        my $x = $input->[$i];

        return [$i, $seen{ $target - $x }]
            if exists $seen{ $target - $x };

        $seen{$x} = $i;
    }
}
[download]

It passes all the tests presented in the thread so far and benchmarks show it's also the fastest.

Translated to Python 3 (Dona eis requiem, smack), it was accepted by LeetCode as

Runtime: 48 ms, faster than 99.40% of Python3 online submissions for T
+wo Sum.
Memory Usage: 15.4 MB, less than 42.44% of Python3 online submissions 
+for Two Sum.
[download]