Question

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What does the function declaration “sub function($$)” mean?

sub t(&@) {
    print @_;
}
t {print 1};

I tried to change &@ to &$ and it will fail.

What’s the lingo for it so that I can search?

Answer 1

&@ is a subroutine prototype. This lets you create syntax similar to the builtin grep function (accepts a BLOCK and then a LIST). The LIST can even be the empty list: ().

&$ when used will force the second argument (which is mandatory) to be evaluated in scalar context. Since there is no second argument in t {print 1}; it will fail to compile.

Read more about subroutine prototypes at: perldoc perlsub.