That would be a prototype, the subject of much discussion on usefulness verging on holy war status.  The Camel Book says the intent behind prototypes "... is primarily to let you define subroutines that work like built-in functions ..." (p. 226).  In this case, it is giving a contextual clue to perl that this sub accepts two scalars.

In reply to Re^3: Increment of ip address by northwind
in thread Increment of ip address by mosh

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