Interesting... Using perl 5.6.1 on solaris:

#!/home/mhough/perl/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %outhash = (abc => 1, def => undef);
my %outhash2 = %outhash1
print Dumper(\%outhash, \%outhash2);

foreach my $str (qw/abc def ghi/)
{
    $outhash{$str}? ($outhash{$str}++) : ($outhash{$str} = 1);
    $outhash2{$str}++;
}

print Dumper(\%outhash, \%outhash2);
[download]

$VAR1 = {
          'abc' => 1,
          'def' => undef
        };
$VAR2 = {
          'abc' => 1,
          'def' => undef
        };
$VAR1 = {
          'abc' => 2,
          'def' => 1,
          'ghi' => 1
        };
$VAR2 = {
          'abc' => 2,
          'def' => '1',
          'ghi' => '1'
        };

Note the subtle difference between key 'def' in the two cases. It would appear that autoincrement on an undef does the magical string autoincrement, leading to a string value of '1', while the code FatVamp offers sets a numeric value of 1.

What does it mean? I don't know, but the two forms do have ever so slightly different results whose difference probably doesn't matter.