$ret{$j}[$i]
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is short for

$ret{$j}->[$i]
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In lvalue context (e.g. on the left of a "="), -> and other dereference operators will autovivify the reference. That means dereference operators will automatically create a variable of the required variable type (e.g. ->[...] will create a hash) and place a reference to it in the variable being dereferenced. This only happens if the variable being dereferenced is undef. In other words,

$ret{$j}[$i]
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is short for

( $ret{$j} //= [] )->[$i]
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when used as an lvalue.