with your code and my data.xml: 
XML::SAX::PurePerl
 doc ()

XML::SAX::Expat
 doc ("Standalone", "", "Encoding", "UTF-8", "Version", "1.0")

XML::SAX::ExpatXS
 doc ()
 decl ("Encoding", "UTF-8", "Version", "1.0", "Standalone", undef)

XML::LibXML::SAX::Parser
 doc ()
 decl ("Version", "1.0", "Encoding", "UTF-8")

XML::LibXML::SAX
 doc ()
 decl ("Version", "1.0")

[download]
It agrees with my own test: XML::LibXML::SAX does not get the encoding while XML:LibXML::SAX::Parser does.
It seems that the parser must be carefully and explicitly selected to get consistent results, rather than letting the factory pass a broken parser.

Update: indeed it seems that XML::LibXML::SAX fails to give the encoding of an utf-8 encoded file while it succeeds with an iso-8859-1. I have no other encoding from xml file right available for another test.

In reply to Re^4: XML::SAX::ParserFactory policy and differences between parser implementations by seki
in thread XML::SAX::ParserFactory policy and differences between parser implementations by seki

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