You're mostly right, but actually LHS || RHS in list context evaluates RHS in list context. Indeed, if you try

perl -we '@bar = (); @bar = @bar || (1, 2, 3); warn "(@bar)";'
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you see that the array is filled with the right values: (1, 2, 3); but that's still useless for indeed when the array isn't empty at the beginning,

perl -we '@bar = (42, 5); @bar = @bar || (1, 2, 3); warn "(@bar)";'
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the or operator returns the length of the array so the array will contain only (2).

Nice trap.