Yes, I am aware of XML::Twig, but it is not suitable to my needs (or at leat I did not see how I could use it, because I need to "patch" an already parsed element to adjust its value during the parsing ans split of a big block of elements that I prefer not to keep in memory)

As you mention yourself in your results, the different SAX parsers are not consistent in regard to the SAX events, at least for XML::SAX::Expat that includes the encoding into start_document() data instead of xml_decl() data or XML::SAX::PurePerl that does not notify xml_decl() at all

Also I do not get the same results as you with my test program and data. Could you check for what file XML::LibXML::SAX manages to give you an encoding? You can see it does not with my utf-8 sample.

data.xml

<?xml version="1.0" encoding="UTF-8" ?>
<root>
    <foo>
        <bar attr="baz">héhé mes 2 €</bar>
        <baz other="dummy"/>
     </foo>
</root>
[download]

test_sax.xml

use strict;
use warnings;
use feature 'say';
#~ use Say; #portability trick for 5.8.8
use XML::SAX::ParserFactory;
use XML::SAX::Writer;

my $input = $ARGV[0] or die "usage: $0 <file.xml> [parser_package]";
$XML::SAX::ParserPackage = $ARGV[1] if $ARGV[1];

my $output; #just for not outputting to STDOUT

my $writer = new XML::SAX::Writer(Output => \$output);
my $handler = new SaxHandler( Handler => $writer );
my $parser = XML::SAX::ParserFactory->parser( Handler => $handler );

say sprintf "parser is %s (%s)", ref $parser, $parser->VERSION ;
$parser->parse_file($input);

{
    package SaxHandler;
    use base 'XML::SAX::Base';
    use Data::Printer {indent=>2};
    use feature 'say';
    #~ use Say; #portability trick for 5.8.8
    
    sub xml_decl {
        my ($self, $decl) = @_;
        say "decl ", np $decl;
        $self->SUPER::xml_decl($decl);
    }
    
    sub start_document {
        my ($self, $doc) = @_;
        say "document ", np $doc;
        $self->SUPER::start_document($doc);
    }
    
    sub start_element {
        my ($self, $el) = @_;
        #~ say "start element " . $el->{LocalName};
        $self->SUPER::start_element($el);
    }
}
[download]

my results:

macbookseb:perl seb$ perl -v
This is perl 5, version 22, subversion 1 (v5.22.1) built for darwin-th
+read-multi-2level[...]

macbookseb:perl seb$ perl test_sax.pl data.xml XML::SAX::PurePerl
parser is XML::SAX::PurePerl (0.99)
document \ {}

macbookseb:perl seb$ perl test_sax.pl data.xml XML::SAX::Expat
parser is XML::SAX::Expat (0.51)
document \ {
  Encoding     "UTF-8",
  Standalone   "",
  Version      1.0
}

macbookseb:perl seb$ perl test_sax.pl data.xml XML::LibXML::SAX
parser is XML::LibXML::SAX (2.0124)
document \ {}
decl \ {
  Version   1.0
}

macbookseb:perl seb$ perl test_sax.pl data.xml XML::LibXML::SAX::Parse
+r
parser is XML::LibXML::SAX::Parser (2.0124)
document \ {}
decl \ {
  Encoding   "UTF-8",
  Version    1.0
}
[download]

Also I do not get the same results as you with my test program and data.

What do you get with my program?

update:

Could you check for what file XML::LibXML::SAX manages to give you an encoding?

When its not utf-8 when its  encoding="ISO-8859-1"

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.