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();
[download]

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.
[download]

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.
[download]

My test code uses:

my $self = { _username => q{Polo},
             _user_tags => [],
             _user_pass => undef,
};
[download]

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>]);
[download]

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>
[download]

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.

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