perl passing hash reference not behaving as expected - perl

Question

(NOTE: I sort of figured this out, see all the way at the end)
It's kind of late and I've been staring at this code for far too long. I finally wrote a short test program to test hashes and passing them by reference and it is not behaving as I expect. I'm sure there is something very simple I'm missing ... can anyone spot it?
#!/usr/bin/perl
use Data::Dumper;
my %hash = ();
print "BEFORE ADDING KEYS\n";
print Dumper (\%hash);
test (\%hash, 10);
print "AFTER ADDING KEYS\n";
print Dumper (\%hash);
sub test {
my %hash = %{$_[0]};
my $number = $_[1];
if ($number == 0) { return; }
print "BEFORE ADDING KEY HASH_REF=$_[0] NUMBER=$number\n";
print Dumper (\%hash);
$hash{$number} = $number;
print "AFTER ADDING KEY\n";
print Dumper (\%hash);
test ($_[0], $number - 1);
}
I expect this code to add the numbers 10 to 1 into my hash, but instead the hash gets wiped out and doesn't contain anything once the test routine finishes recursing. What am I missing? Here is the output:
BEFORE ADDING KEYS
$VAR1 = {};
BEFORE ADDING KEY HASH_REF=HASH(0xdb82fd0) NUMBER=10
$VAR1 = {};
AFTER ADDING KEY
$VAR1 = {
'10' => 10
};
BEFORE ADDING KEY HASH_REF=HASH(0xdb82fd0) NUMBER=9
$VAR1 = {};
AFTER ADDING KEY
$VAR1 = {
'9' => 9
};
...
BEFORE ADDING KEY HASH_REF=HASH(0xdb82fd0) NUMBER=1
$VAR1 = {};
AFTER ADDING KEY
$VAR1 = {
'1' => 1
};
AFTER ADDING KEYS
$VAR1 = {};
Changing this line:
$hash{$number} = $number;
to:
$_[0]->{$number} = $number;
made everything work as expected. Why does the first statement modify a fresh local %hash when I would expect this local %hash to be pointing to the same de-referenced hash reference I originally passed into the routine?

Answer 1

Everything works as intended. You first statement in the test sub makes a copy of passed hash:
my %hash = %{$_[0]};
To mutate passed hash, you should work with hashref, like:
my $hashref= $_[0];
$hashref->{key} = 'val';
This approach will change original hash, not it's copy.