Well that's (worst case) O(n) right there. :-)

That's what linear means.

Does != even have a meaning in list context ?

Edit: It doesn't force scalar context, but seems to compare array lengths. See :

sub zero { return (0, 0) }
print +(zero() == 2) ? "scalar\n" : "list\n";
my @a = (0, 1);
my @b = (1, 0);
my @c = (0, 2 , 1);
print +(@a == @b) ? "same length\n" : "not\n";
print +(@a == @c) ? "same length\n" : "not\n";
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Anyway, I think perl doesn't actually build the lists and directly optimize that to the array size, which should make it constant time.