Perhaps grep might work better here?

my @a = (1,2,3,2,1);
my %seen;
my @unique_a = grep {!$seen{$_}++} @a;

print "A:      $_\n" for (@a);
print "Unique: $_\n" for (@unique_a);
[download]

Assuming you wanted to keep the first occurance, this will preserve the original order.

Or you can use map to create your %a hash.

my @a = (1,2,3,2,1);
my %a = map {$_=>1} @a;
my @unique_a = keys %a;

print "A:      $_\n" for (@a);
print "Unique: $_\n" for (@unique_a);
[download]

-Blake