++ to hippo and dave_the_m for spotting the mistake.

For your information, perl would have warned with use warnings (unless there was a "b" key somewhere in the hash):

Use of uninitialized value in numeric comparison (<=>)
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That's because if $interface_bytes{"b"} is empty, perl will return undef for the missing value, and the comparison operators (<=> and cmp) will warn when one of the values is undef