Since it's my code I guess I ought comment on it:

opendir my $d, $dir;
[download]

This should really have an or die for sanity and a more descriptive variable name than $d such as $dir_handle.

my @f = sort { -M $b <=> -M $a } readdir $d;
[download]

This sorts all the files in the directory by modification time in reverse chronological order, again, a more descriptive variable like @file_list would be better suited. It should also only grab files so a grep should be added i.e

my @file_list =  sort { -M $b <=> -M $a }
                 grep -f, readdir $dir_handle;
[download]

And finally:

unlink @f[-0,1] if @f > 20;
[download]

This removes the two oldest files if there are more than 20 elements in @f. This code is somewhat broken as the pathnames are not fully qualified, which could be done like so:

use File::Spec 'catfile';

unlink map catfile($dir, $_), @f[0,1]
  if @file_list > 20;
[download]

Also the negated zero in the original code was just a blip from knocking out a quick bit of code in the CB. So, in summary, this more thought out code would be better:

use File::Spec 'catfile';

opendir my $dir_handle, $dir
  or die "Couldn't open '$dir': $!";

my @file_list =  sort { -M $b <=> -M $a }
                 grep -f, readdir $dir_handle;

unlink map catfile($dir, $_), @f[0,1]
  if @file_list > 20;
[download]