exec is not OA_DANGEROUS

exec is not OA_DANGEROUS

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

When exec fails, it only ever returns a target unused elsewhere, so it
does not necessitate temp copies in list assignment.