my @sorted = sort { $a->{num} cmp $b->{ num } } @$data;

##</code><code>##

#!/usr/local/bin/perl -w
use strict;

my $data =
[
    {
        num     => 'OF1234',
        title   => 'title OF1234',
    },
    {
        num     => 'AF1234',
        title   => 'title AF1234',
    },
    {
        num     => 'AF1234',
        title   => 'title AF1234',
    },
];

my %saw;
my @sorted =
    grep { ! $saw{$_->{num}}++ }
    sort { $a->{num} cmp $b->{ num } }
    @$data;

print join ', ', map { $_->{num} } @sorted;