'|' is left associative, and the left argument is evaluated first

Left-associative means that

a | b | c
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is equivalent to

( a | b ) | c
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and not

a | ( b | c )
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It says nothing about the order in which a, b and c are evaluated in relation to each other. In fact, the operand evaluation order is undefined for most Perl operators including the "|" operator.

Perl is free to evaluate

a | b | c
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as follows:

push @stack, c;
push @stack, b;
push @stack, a;
push @stack, pop(@stack) | pop(@stack);
push @stack, pop(@stack) | pop(@stack);
pop @stack;
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The left pipe is evaluated before the right pipe, so the operator is left-associative, but note how c is evaluated before b, which is evaluated before a.

Your code is relying on undefined functionality. It's not guaranteed to work. You'd have to switch to || if you wanted predictable results. || is guaranteed to evaluate its LHS argument before its RHS argument due to its short-circuiting nature.

That's the official line, anyway. In reality, |'s LHS arg is always evaluated before its RHS, and I don't see that ever changing.