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my @a = (1,2,3,2,1);
my %seen;
...

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

my @a = (1,2,3,2,1);
my %a = map {$_=>1} @a;
...
print "A:      $_\n" for (@a);
print "Unique: $_\n" for (@unique_a);