Those are prototypes, documented in perlsub. They are used reduce code clutter and mimic the behavior of builtins by changing how the compiler parses subroutine calls.

Most Perl programmers new to prototypes instantly assume they're like other languages' prototypes, i.e. they are for declaring how many and what type of parameters a function expects. This is a misuse of them, as it generally causes problems and is quite often bypassed entirely anyways. See FMTEYEWTK about Prototypes. The article was long ago removed from O'Reilly's site, but it still remains relevant.