why is it not a syntax error?

In my opinion, it is a syntax error on 5.12. When I add a print to your try.pl file, I get the same error message as you do, and it does not print "foo". On 5.8.8, I get no error message and it does print "foo".

#!/usr/bin/perl
use strict;
use MyPackage;

MyPackage::close_cnx();
print "foo\n";
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As an aside, you can promote warnings to errors like this:

use warnings FATAL => 'all';
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