Short version: uncomment lines 86-87, patch follows. I was hoping I wouldn't need them, but I was wrong. This has been tested, and should produce the correct output now.
--- mandelbrot.pir 2005-12-22 11:51:15.000000000 -0500
+++ mandelbrot-new.pir 2005-12-22 11:51:07.000000000 -0500
@@ -83,8 +83,8 @@
Zr = Tr
Zi = Ti
# if (Zr*Zr+Zi*Zi > limit*limit) break;
-# $N1 = Zr * Zr
-# $N2 = Zi * Zi
+ $N1 = Zr * Zr
+ $N2 = Zi * Zi
$N1 += $N2
if $N1 > limit goto IBRK
inc i
