Should be:

#!/usr/bin/perl
print "Content-type: text/html\n\n";
print "NOTHING TO SEE HERE!\n";
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If you don't print the header you will get the '500 Internal Server Error'

Also, what Hostgator support are saying about the PHP file is rubbish, but if you do want to test it then put just this in your index.php:

<?php echo '<p>Hello World From the PHP File</p>'; ?>
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The first line of the PHP must contain <?php ...