Perl does not guarantee what order the two operands of a binary operator will be evaluated in.

Operand evaluation order is documented and thus guaranteed for about half the ops*. And while it's not guaranteed for the other ops, it never changes (left-to-right for all ops (including **) except assignment ops).

* — Going from memory, operand evaluation order is documented for:

In reply to Re^2: $1 not "freezing" in an addition by ikegami
in thread $1 not "freezing" in an addition by grondilu

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