One of the "features" of the Schwartzian transform is that you don't have to create any named temporary variables. So instead of using %saw or $prev you could do

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

my @sorted =
    sort { $a->{num} cmp $b->{ num } }
    values %{ +{ map { ($_->{num},$_) } @$data }};

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