The "EXPR" in "(split ...)[EXPR]" must be

an expression that returns a list of indexes.

In your code, you have

an expression that returns source code that once compiled and executed returns a list of indexes.

Solution:

my $prodKey = join("|",(split/\|/,$$prodRef[$a])[@arrofIndices]);
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By the way, $$prodRef[$a] is more readable as $prodRef->[$a]. See Dereferencing Syntax.