Do you mean this one?

$Old_MacDonald = q#print #; 
$had_a_farm = (q-q:Just another Perl hacker,:-);
s/^/q[Sing it, boys and girls...],$Old_MacDonald.$had_a_farm/eieio;
[download]

As far as I can tell, the first (currently) recorded appearance of this can be found on this announcement regarding what I presume is the first edition of a certain book in most of our collections.

(Note: I've written as three lines because the two-line version wrapped oddly.)

--f