You could refine that to have an out list rather than an out hash. Every value you store in it has the value 1!

my %multiples;
my @out;

for (@in) {
    if ($multiples{$_} == 1) {
        push @out, $_;
    }
    $multiples{$_} = 1;
}

print "@out";
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Now since it contains only one statement, you could write the if in suffix form. You could use ++ instead of setting the count to 1 in the unconditional statement.

That gets down to:

my %multiples;
my @out;

for (@in) {
    push @out, $_  if ($multiples{$_}++ == 1)
}

print "@out";
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That's tight enough that you can see how merlyn's form works. Just use the built-in looping mechanism of grep instead of your own foreach loop/push. It's the same thing.

He used pre-increment == 2, which you'll note is the same thing as post-increment == 1. The latter is more directly equivilent to what you had originally, but the pre-increment is arguably more efficient (though I don't know if measurably faster in Perl).