When a pattern is compiled, all the captures are allocated fixed indices: so for example when the following pattern is compiled, the indices shown are assigned.

/(....)...(...)...(??{foo()})....(...)/
 ^        ^                     ^
 1        2                     3
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In particular, the last capture is index number 3.

Each time the pattern is run, foo() might return a string or qr// which has a different number of captures; and which might recursively also include /(??{...})/ and further nested captures. This would make it nearly impossible to guess how to refer to the final capture in the outer pattern, unless we just always call it $3.

Dave.