my @nums = qw(2 1 3 5 4 5 4 3 2 1);     # order is not important

my @unique;     # new list of unique elements
my %seen;       # numbers i have seen so far

foreach my $x ( @nums ) {   # check each number
    if ( ! $seen{$x} ) {    # skip if we already saw this one
        $seen{$x} = 1;      # note we have seen this one now
        push @unique, $x;   # and store to new list
    }
}
## TODO: do something with @unique