The thing is a "prototype", see the "sub" keyword that gives away the fact that it's not a function call, but a function declaration.

Sub prototypes were an experiment in Perl, and most experienced Perlers seen to agree we'd better classify it under the "mostly failed experiments".

See my archived copy of Tom Christiansen's old (but excellent and still very relevant) comprehensive article about prototypes, in FMTEYEWTK about prototypes.