Holy obfuscation, Batman!

That seems to work. You could choose to use that (and hopefully leave a comment above it in your script). But you can get rid of some cruft there.

First, as mentioned before, the dummy dim on $b is arbitrary. Using xchg when you already got to pick the dims is silly.

which( maximum( ($a->dummy == $b) ) != 0)
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Maximum is a good choice. Like summation, it folds over a dimension in a way that works like logical or. But the inequality is unnecessary. All non-zero values are true. which already handles this.

which( maximum( $a->dummy == $b ) )
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Thats about as good as any solutions I could give you. You'll still need a comment, but its a little easier on the eyes.

For comparison purposes, here's the solutions I was expecting:

# The one I basically handed you. Catches duplicates.
$c = which( $a == $b->dummy ) % $a->dim(0);

# Switching up the dims. Basically same as above.
$c = which( $a->dummy == $b ) / $b->dim(0);

# Using summation. Like yours, ignores duplicates.
$c = which( sumover( $a->dummy == $b ) );
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They are no more understandable than your solution (once trimmed).