Here's uniq from List::MoreUtils:

sub uniq (@) {
    my %h;
    map { $h{$_}++ == 0 ? $_ : () } @_;
}
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I think this is equivalent to your "my %h; @uniq = grep { ! $h{$_}++ } @in;". What's important about both of them is that they don't do this:

sub bad_uniq {
    my %h;
    @h{@in} = ();
    return keys %h;
}
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...which may be more succinctly (idiomatically) written as "keys %{{ map { $_ => 1 } @in }}".

The difference is that keys will not return what was in the list to begin with but rather whatever those things come out as after being stringified. The solutions using map and grep both will stringify the stuff you feed them, but it's the stuff that's returned, and not the stringy leftovers that were used to tell which were duplicates.

Update with a demonstration:

use List::MoreUtils 'uniq';
use Data::Dumper;

my $aref = [ 1, 2 ];
my @aref_duplicated = ( $aref, $aref, $aref );

my @u1 = uniq( @aref_duplicated );
my @u2 = keys %{{map {$_=>1} @aref_duplicated}};
print Data::Dumper->Dump( [ \@u1,         \@u2         ],
                          [ '*from_uniq', '*from_keys' ] );
__END__
@from_uniq = ( 
               [ 
                 1,
                 2
               ]
             );
@from_keys = ( 
               'ARRAY(0x8153c28)'
             );
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