I modified your code below to see how XML::Simple handles this, is seems to do the same thing, i suspect its one of the underlying modules possibly XML::Parser thats causing this:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use XML::Simple;
my $xml = XMLin(qq{<root t="space tab\tnewline\nend"/>});
print Dumper $xml;
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output:

$VAR1 = {
          't' => 'space tab newline end'
        };
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