local $XML::Simple::PREFERRED_PARSER = 'XML::Parser'; is misplaced. You call load after the local falls out of scope, and thus after the local undid the assignment.

The point of specifying PREFERRED_PARSER is to choose the fastest parser available for XML::Simple and to avoid buggy XML::SAX::PurePerl. Since you get the the encoding error when you don't specify PREFERRED_PARSER, you must be defaulting to XML::SAX::PurePerl. I take it out of my XML::SAX configuration file so it never gets selected.

You are also using incorrect prototypes. You say your functions take no arguments, but that's obviously not true.

Fixed:

package ETTX;
use strict;
use warnings;
use XML::Simple;
sub new { #scalar file name
    my $class = shift;
    my $self = 
        {
            ettxFile => '',
            ettx => {},
        };
    bless $self, $class;
    $self->load(@_) if @_;
    return $self;
};

sub load { #scalar file name
    my ($self, $ettxFile) = @_;
    print "loading $ettxFile";
    open(my $fh, '<', $ettxFile);
    binmode($fh);
    local $XML::Simple::PREFERRED_PARSER = 'XML::Parser';
    my $xml = XML::Simple->new();
    $self->{ettx} = 
        $xml->XMLin($fh, 
            ForceArray => ['map'], KeyAttr => {}, 
        )
    ->{table};
    $self->{ettxFile} = $ettxFile;
    1;
}
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