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:

• && || // and or xor ?:
• = **= += *= &= <<= &&= -= /= |= >>= ||= .= %= ^= //= x=
• , => (in scalar context)
• .. ... (in scalar context)