Neither methods generate a warning for me, using perl 5.8.4.

$ perl -wle '$outhash{$ARGV[0]}++; print $outhash{$ARGV[0]}' foo
1
$ perl -wle '$outhash{$ARGV[0]}
             ? ($outhash{$ARGV[0]}++)
             : ($outhash{$ARGV[0]} = 1);
             print $outhash{$ARGV[0]}' foo
1
$
[download]