I agree that the docs could be clearer. Setting this variable causes code to die badly if the fast module is not installed.

Here is code that I use to check if the fast module is installed. If it is installed, the code is happy, and it just uses the fast module. If it is not installed, it quietly falls back to the slow module:

use English qw($CHILD_ERROR);
use XML::Simple;

# If a fast parser for XML::Simple is installed, then use it.
my $parser = 'XML::LibXML::SAX';
my $cmd    = "perl -M$parser -e 1";
my $output = qx($cmd 2>&1);
unless ($CHILD_ERROR) {
    $XML::Simple::PREFERRED_PARSER = $parser;
}
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