That doesn't seem to work:

%  perl -we 'sub fn {print "$a$b\n"} print(sort(fn(0,0,1,2)))'
Unquoted string "fn" may clash with future reserved word at -e line 1.
00
10
21
0012
[download]

I can eliminate the warnings by choosing a function name that isn't all-lowercase but, as the output demonstrates, the function is still called as a comparator, not a modifier of the input list.

Update: I was using perl 5.6.1. The parentheses trick works with 5.8.