use strict;

my %a;

$a{"abc"} ++;
print $a{"abc"};

##</code><code>##

use strict;

my %a;

check();
print "now set abc to undef\n";
$a{"abc"} = undef;
check();

sub check {
    print "defined\n" if (defined($a{"abc"}));
    print "exists\n" if (exists($a{"abc"}));
}