It still doesn't make any sense. I see now though that f is actually getting called in both cases (or would be if there is more than one element). In the runs that return 0, it's called before the sort. In the runs that return 1, it's called as the sorting subroutine. The fact that whitespace indicates precendence really bugs me though. I feel like it should always return one or the other (probaby 1), and that you should be forced to call the subroutine with &.

To get closer to my original code:

#!/usr/bin/perl

use strict;
use warnings;

my @list = (1, 2, 1);
print join ',', sort (unique (@list));
print join ',', sort (unique(@list));

sub unique { 
  my %new = ();
  return grep (!$new{$_}++, @_);
}
[download]

Chromium:~ lexicon$ perl -l little2.pl 
Unquoted string "unique" may clash with future reserved word at little
+2.pl line 7.
1,2,1
1,2
[download]

Chromium:~ lexicon$ perl -l -MO=Deparse little2.pl 
Unquoted string "unique" may clash with future reserved word at little
+2.pl line 7.
BEGIN { $/ = "\n"; $\ = "\n"; }
use warnings;
use strict 'refs';
my(@list) = (1, 2, 1);
print join(',', (sort unique @list));
print join(',', sort(unique(@list)));
sub unique {
    use warnings;
    use strict 'refs';
    my %new;
    return grep((!$new{$_}++), @_);
}
little2.pl syntax OK
[download]

I supose here, in the unique as sort-on-subroutine case, unique always returns a two element list which is probably interpreted as 1 and so leaves the list in order. This ends up looking like the unique subroutine is completely ignored, which drove me nuts for a couple hours.