pat, I removed the dim line from your code and moved the $a = $ = 1 up a bit and got 80:
Code:

Function A($)
Do
 Do
  $a=$a + $ mod 10 * ($ mod 10)
  $=$/10
 Until $=0
 $=$a
 $A=$=1
Until $<5
EndFunction


$=1 you know, it's evaluation.
$=1 returns true (or 1) if $ equals 1. otherwise it returns false (or 0).

thus $A = $ = 1 resets $A to zero always except when $A should return 1 (happy number).
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