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]
<Reveal this spoiler or all in this thread>

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: LeetCode Problem #1 - Two Sum - Improve and/or discuss. by choroba
in thread LeetCode Problem #1 - Two Sum - Improve and/or discuss. by kcott

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