plperl: sort trapped by operation mask

I'm getting 'sort trapped by operation mask' from plperl in postgresql
7.4.8. I understand what this message means, but can some one out there
tell me why sort is privileged? Or, is it just untrusted in the context
of s///e ?

The relevant code snippet will be hard to understand, but it is:

s(at)\\[([^\\]]+)\\](at){my %aa=map {$_=>1} split(//,$1); "[".join("",sort
keys %aa)."]"}(at)eg;

This works fine when 'sort' is removed.

Thanks,
Reece

--
Reece Hart, http://harts.net/reece/, GPG:0x25EC91A0