use strict;
use warnings;

my @items = ( 1, 3, 3, 3, 5, 5, 5, 5 );
my @frequency = @{ frequency( \@items ) };

print "@frequency\n";

sub frequency {
    my $items_ref = shift;
    my %counter;
    $counter{$_}++ for @{$items_ref};
    return [ map { $counter{$_} } sort { $a <=> $b } keys %counter ];
}