Just did this the other day. The key is to remember that sort's curly block is essentially an anonymous sub, so what you put there has to be executable code. In the case of a sort subroutine, this means you need to do something like this:

my $dir = shift; 

my $sort_sub = $dir eq 'up' ? sub {$a <=> $b} : sub {$b <=> $a};
my @items = (3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5);
print "@{[sort { $sort_sub->() } @items]}\n";
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Which gives us

$ perl sample up
1 1 2 3 3 4 5 5 5 6 9
$ perl sample down
9 6 5 5 5 4 3 3 2 1 1
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Note that the code in the sort curlies calls the callback itself.