my @nums = qw(2 1 3 5 4 5 4 3 2 1);

@nums = sort @nums;
my @unique;

my $previous;
foreach my $current ( @nums ) {     # check each number
    next if $current eq $previous;  # duplicate!
    push @unique, $current;         # store to new list
    $previous = $current;     # 'current' becomes 'previous'
}
## TODO: do something with @unique