Your first version is different from the others because you give the hash itself as parameter instead of a reference to the hash. The parameter list of a subroutine invocation is an array context. But a hash in array context is simply an array with key value pairs. So with your first version

XMLout(%$self , XMLDecl => 1 ...

# is nearly* equivalent to

XMLout(_username, Polo, _user_tags, [],_user_pass, undef , XMLDecl => 
+1 ...

#  * not exactly equivalent because the order of the hash 
#    contents is random
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Your second version seems to be identical to your third version (at least I couldn't spot a difference at first glance, but I didn't test it), could it be that you made some typing or copying error somewhere?

It's important to provide sufficient information so that your problem can be reproduced. You have not stated what $xml is. I've assumed this is an XML::Simple object: it could be a subclass of XML::Simple or, indeed, any class providing an XMLout() method. Do let us know if my assumption is wrong! So, my test code (xml_simple_args.pl) starts with:

use strict;
use warnings;
use XML::Simple;
my $xmlfile = q{xml_simple_args.xml};
my $xml = XML::Simple->new();
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It's important to post the code you've actually tested. The assignment to $self produces these messages:

$ xml_simple_args.pl
Bareword "Polo" not allowed while "strict subs" in use at ./xml_simple
+_args.pl line 9.
Execution of ./xml_simple_args.pl aborted due to compilation errors.
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and after quoting Polo:

$ xml_simple_args.pl
Can't modify constant item in scalar assignment at ./xml_simple_args.p
+l line 11, near "],"
Bareword "_user_tags" not allowed while "strict subs" in use at ./xml_
+simple_args.pl line 9.
Execution of ./xml_simple_args.pl aborted due to compilation errors.
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My test code uses:

my $self = { _username => q{Polo},
             _user_tags => [],
             _user_pass => undef,
};
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I was able to reproduce the first error you posted. The third line of the XML::Simple documentation SYNOPSIS has:

my $xml = XMLout($hashref [, <options>]);
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I changed %$self to $self and this worked without error. jethro has provided information about this (above).

I was unable to reproduce your second error: the code you posted worked just fine.

I was unable to reproduce your success with the third version you posted; however, after terminating each statement with a semi-colon, this worked fine as well.

Each successful run produced the same output:

$ cat xml_simple_args.xml
<?xml version='1.0' standalone='yes'?>
<opt>
  <_user_pass></_user_pass>
  <_username>Polo</_username>
</opt>
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Lessons learned from this exercise:

• Post sufficient code so others can reproduce your results (e.g. state what class you're using).
• Post the code you actually tested.
• Read the documentation.
• Add use strict; and use warnings; near the start of your code.

Thanks, your comment made me, finally, realize the error in my code

First I want to apologize for not posting correct code, but I am unable to disclose the code i am workning with right now

So, the problem rised when in the object I had..

sub new
{
  ...
  $self = {
      ...
      _IP => undef,
      ...
  }
  bless ($self, $class);
  return $self;
}

sub set_ip
{
  my ($self,$ip) = @_;
  $self->{_IP} = new Net::IP($ip) or die (Net::IP::Error());
}
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The problem here is that Net::IP does not overload the method '==' so XMLout explodes at line 1406 in Simple.pm.

if(grep($_ == $ref, @{$self->{_ancestors}}));
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You can see this line is trying to look for references to the present node (circular references)

So either a) overload '==' at XML::Simple b) Instead of using Net::IP use simple strings c) comment lines 1405 and 1406

Thanks

And the question is, Why the different behaviour if all the options I tried are basically the same?

Because in real life (unlike the sample you've shown here), they are different, they are not the same