Okay, lemme know if I'm wrong here, but:

   $lines[0]       = [ 3,2,1 ];
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The brackets are forming an array context. You're assigning an array to the array member

   print    $lines[0][ 3,2,1 ];
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I am assuming what is going here is that the array dereference is looking for a numerical value to figure out where to look in the array. You get the same error here:

  print scalar(4,5);
jaybonci@starlite:~/perl/pm$ ./array.pl 
Useless use of a constant in void context at ./array.pl line 6.
5jaybonci@starlite:~/perl/pm$
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Looking at the comma operator in the perlop page, this makes sense. The values are evaluated one by one.

It wouldn't make sense for perl to be looking for an array in a dereference operator, because you need a numerical value. Why would it map those values or try to return a sub array based on those values. The execution order says to resolve the commas first.

Like I said, I could be way off.

    --jb