How to modify HEAP variables in subs...

Hi, please sorry, i`m newbie in POE and have a lot of 
problems...
Why doesn`t variables of HEAP changed?
There is a little chunk of program(2 subs):

sub one {

$_[HEAP]->{VARIABLE}=0;
for(1..4)
{
$_[KERNEL]->yield("two");
}
print "VARIABLE: ",$_[HEAP]->{VARIABLE};

}

sub two {
$_[HEAP]->{VARIABLE}++;
}

It will print "VARIABLE: 0", but why? I`ve updated it in 
'two', it must be 1(cause of ++).
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