First of all, XML::LibXML (and thus XML::LibXML::SAX) come with Strawberry Perl. So you do have the module.

In your Perl installation, there's a file named XML/SAX/ParserDetails.ini that says which SAX parsers you have installed. XML::Simple uses the first parser listed in this file. For you, that parser is XML::LibXML:SAX.

The problem is that PAR::Packer doesn't know this, so it didn't include XML::LibXML:SAX in the distributable package. You'll need to notify PAR::Packer of this when you create the distributable package.

pp -M XML::LibXML::SAX ...
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Updated due to a better understanding of the question.

That won't pack all the dlls required like these

c\bin\libxml2-2_.dll
c\bin\libiconv-2_.dll
c\bin\zlib1_.dll
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depends.exe /c /f:1 /pb /ot:temp.txt ... ...\perl.exe ...\foo.pl ...

I've never used pp. Feel free to provide the more complete command.

BEGIN { $ENV{XML_SIMPLE_PREFERRED_PARSER}='XML::Parser'; }
use XML::Simple qw/ XMLin /;
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$XML::Simple::PREFERRED_PARSER = 'XML::Parser';
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would be more straightforward and less error-prone than

$ENV{XML_SIMPLE_PREFERRED_PARSER} = 'XML::Parser';
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---

Whichever method you use, it doesn't need to be done before the module is loaded; just before XMLin is called. Safer:

use XML::Simple qw( XMLin );

sub parse_xml {
   local $XML::Simple::PREFERRED_PARSER = 'XML::Parser';
   return XMLin(@_);
}

my $xml = parse_xml(...);
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would be more straightforward and less error-prone than

Nonsense

Whichever method you use, it doesn't need to be done before the module is loaded; just before XMLin is called.

true

Safer:

Nonsense

After packing and moving to a system without perl the program fails that is cannot find XML::LibXML::SAX, it seems PAR::Packer adds this in as a requirement.

After doing what?