$x[0][1] = 12;
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is the syntactic-sugar equivalent of

$x[0]->[1] = 12;
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(IIRC, in Perl 4, this was the required invocation.)

And the whole sub-array as

$x[0] = \@subarray;
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You might also think about it like this:

@x = ( ["some", "sub", "array"], ["next", "sub", "array] );
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which is making anonymous arrays, and assigning the lot to @x. And these can be nested as deeply as you have memory for (AFAIK, but that would be a good exercise to check).

Update:

Yes, it seems that simply nesting empty arrays keeps going until it runs out of memory, starts swapping, and in my case, is killed by the OS.