The section "ENVIRONMENT" of the docs explain why this might happen. If you look at line 369 of XML::Simple it has:

eval { require XML::SAX; };      # We didn't need it until now
if($@) {                         # No XML::SAX - fall back to XML::Par
+ser
    if($preferred_parser) {      # unless a SAX parser was expressly r
+equested
    croak "XMLin() could not load XML::SAX";
}
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So it would seem that in the script that fails it is defining what parser to use - have a look for the environment variable 'XML_SIMPLE_PREFERRED_PARSER' or the package variable $XML::Simple::PREFERRED_PARSER in the script.