What are Data::Dumper()'s side effects?

Question

If I comment out the Dumper($cmd_string) then the while loop is never taken.

What side effects does Dumper() have on $cmd_string?

Here is what $cmd_string is before the sub call:

VAR1 = {
    'The Java Runtime Library' => {
        'apt-get install -y' => 'sun-java6-jre'
    }
};


sub installPackages
{
    my $cmd_string = shift;
    my %rc_hash;

    my $rc;

    Dumper($cmd_string);

    for my $desc (keys %{$cmd_string})
    {
        while (my ($cmd, $arg) = each %{$cmd_string->{$desc}})
        {
            print "system($cmd $arg)\n";

            $rc = system("$cmd $arg");

            if ($rc)
            {
                $rc_hash{$desc}{$cmd} = '';
            }
        }
    }
    return \%rc_hash;
}

If I run the Perl debugger without the Dumper() and use the x command on $cmd_string then it works, but if I just step through the code it does not work.

This is after only stepping though the code at the end of the sub

  DB<3> x $cmd_string
0  HASH(0x2769550)
   '' => HASH(0x2769880)
        empty hash
   'The Java Runtime Library' => HASH(0x25cc2a0)
      'apt-get install -y' => 'sun-java6-jre'
  DB<4> x $cmd_string->{$desc}
0  HASH(0x2769880)
     empty hash

Now, if I x $cmd_string before the for loop I get this at the end of the sub

main::installPackages(msi.pl:1979):       return \%rc_hash;
  DB<3> x $cmd_string
0  HASH(0x1125490)
   'The Java Runtime Library' => HASH(0xf852a0)
      'apt-get install -y' => 'sun-java6-jre'

Answer 1

The each iterator over hashes uses a hidden per hash variable to keep track of where it is in the hash. My guess is that the code used to generate the $cmd_string hash also uses each but is not iterating to completion.

To reset the each iterator, place keys %{$cmd_string->{$desc}}; before your while loop. Calling keys in void context is the standard way of resetting the hash iterator.

Alternatively, just use for my $cmd (keys %{$cmd_string->{$desc}}) { and then create the $arg variable inside the loop.

The reason why using Dumper() fixes the problem is that Dumper most likely calls keys on the hash, thereby resetting the iterator.