This is the easy way, but it ignores the requirement to print "and" before the last known item. You would print "I know about this,that,the other,the last thing.", not "I know about this, that, the other, and the last thing.".
It FIGURES that I would overlook that. Thank you, BlaisePascal, for spotting it. It's nice comments like yours that will help to keep me on my toes. :-)
Well, in an attempt to redeem myself, here's an oversimplified (and crude)fix:
@a = ('this', 'that', 'the other', 'the last thing');
$lastone=pop(@a);
$tmpstring=join(',',@a);
$string = "I know about $tmpstring, and $lastone.";
print $string;