With no change to the idiom, I'd write that as,

my %saw;
my @out = grep { ! $saw{$_}++ } @in;
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But that extends the lifetime of %saw. If later in the same (or an inner) block, you need to use the same construct, you have to use a different name for the hash, or do a %saw = () - which will then cause errors if you remove the first construct (until you my the newer construct).

I like to write it as:

my @out = do {my %saw; grep !$saw{$_}++, @in};
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which doesn't leak the name of the temporary array.