This is the way I've usually seen it done. I was curious, so I ran a benchmark against the two. 
use Benchmark;

my @list;
for ( 0..9999 )
{
    push @list, sprintf "%d", 100 * rand ;
}


timethese(
    1000, 
    {
        'keys_map' => sub { my @uniq = keys %{{ map {$_ => 1} @list }}
+; },
        'grep_seen'    => sub { my %seen; my @uniq = grep ! $seen{$_}+
++, @list; },
    }
);

[download]
Yields the following output.

Benchmark: timing 1000 iterations of grep_seen, keys_map...
 grep_seen: 13 wallclock secs (11.17 usr +  0.00 sys = 11.17 CPU) @ 89
+.52/s (n=1000)
  keys_map: 30 wallclock secs (29.28 usr +  0.00 sys = 29.28 CPU) @ 34
+.15/s (n=1000)

[download]

In reply to Re: Re: Perl Idioms Explained - keys %{{map{$_=>1}@list}} by LanceDeeply
in thread Perl Idioms Explained - keys %{{map{$_=>1}@list}} by broquaint

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