Use a global flag to say you are done:

use strict;
use warnings;
use File::Find;

my $base = '/my/working/directory';
my $done = 0;

finddepth(\&no_old_dir_please, $base);

sub no_old_dir_please {
   $done ||= (-M $_) <= 30;
   $File::Find::prune = $done;
}
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Update: Read the docs before replying next time :)

Note, not tested