use Data::Dumper;
use strict;
use warnings;

my %hash;

assignWithDupes([
    "one" => "1",
    "two" => "2",
    "one" => "3",
    "three" => "4",
    "two" => "5",
    "one" => "6",
],
\%hash);

print Dumper(\%hash);

sub assignWithDupes {
    my ($arr, $hash) = @_;
    my ($i, $j, $key, $val);
    
    for ($i = 0; $i < $#$arr; $i += 2) {
        $key = $arr->[$i];
        $val = $arr->[$i+1];
        
        if (!defined $hash->{$key}) {
            $hash->{$key} = $val;
            next;
        }

        for ($j = 2; defined $hash->{"${key}_$j"}; $j++) {}
        $hash->{"${key}_$j"} = $val;
    }
}