array elements are not unique, but hash keys are, so a common approach is to use the uniqueness of hash keys to resolve your array.

if you want to preserve the original order, you can do this:

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
[download]

if you want to sort the resulting list, you can do this:

my @nums = qw(2 1 3 5 4 5 4 3 2 1);    # order is not important
my %seen = map { $_ => 1 } @nums; # build a hash; keys will be unique!
@nums = sort keys %seen;       # replace old list with new sorted list
## TODO: do something with @nums
[download]

another approach, as rsriram demonstrated above, is to sort first, then loop and look for repeating values:

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
[download]

(updated to fix some silly issues with untested code)

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