or, for slightly better performance (IIRC), ditch map in void context, and use

$counts{$_}++ for @test;
[download]

or (for us antediluvian 5.004 types)

for (@test) {$counts{$_}++}
[download]

I was hoping there was a Schwartzian Transform analog for this, just for the fun of running the whole thing into one line, but I can't come up with one.

Update:
No sooner posted than obsolete...

@redundant = grep {++$counts{$_} > 1} @test;
[download]

Oops!

@redundant = grep {++$counts{$_}==2} @test;
[download]

and a pox on foolish monks who post without full testing...

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If God had meant us to fly, he would *never* have give us the railroads.
    --Michael Flanders