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my %a = (foo => undef);
print "existed = ".(() = delete $a{foo})."\n";
...
__END__
existed = 1
notexisted = 1

my %a = (foo => 1, bar => 2, baz => 3);
$" = ',';
...
__END__
existed = 1
notexisted = ,2,3

use Devel::Size qw(size total_size);

...
c: 16
d: 16
e: 29