Dear Monks,
I have encountered a quite crazy issue with Perl. Please look at the following example:

{
    package FooObj;
    sub new      { bless { _count => 1,
                           _data  => 0 }, $_[0] }
    sub add_data { 
        
        my $self = shift;
        my $data = shift if (@_);

        print "\nCalled: data value is " . ($data || "NONE") . "\n";

        if (defined $data) { die "\nError: data does not look like num
+ber\n\n" if ($data !~ /^\d+$/); }

        if ($self->{_count}) {

            $data = 5 if (!defined $data);
            $self->{_data} += $data;
            $self->{_count}--;

            return($self->{_data});

        }
        
    }

}

my $bar = FooObj->new;
while(my $val = $bar->add_data()) {

    # do something here

}
[download]

In the above example, the method add_data() should be called twice. Both times, the method is called with no argument, therefore the private variable $data should be undefined. Later on, inside the method, if $data was not defined, it gets assigned a value of 5.

Here comes the weird part. If you run the above code, it outputs:

Called: data value is NONE

Called: data value is 5

It looks like, the second time add_data() gets called, $data did not go out of scope and its previous value is still there.
However, if I replace the line my $data = shift if (@_); with my $data = shift;, I get the expected output:

Called: data value is NONE

Called: data value is NONE

Can someone please explain me what is going on? Is this a Perl bug or am I missing something??

Thanks!!!

Cheers,
Daniel