$myhash{$val_1} = $val_2 // '';

##</code><code>##

$myhash{$val_1} = defined $val_2 ? $val_2 : '';

##</code><code>##

foreach $val_1 (keys %myhash) {
   no warnings 'uninitialized';
   print "myhash{$val_1} = $myhash{$val_1}\n";
}