This may shock you, but: historically, Perl functions did not have declared arguments. Like, at all. The function just got arguments implicitly passed to it as an array (@_), and parsing that into individual variables was up to the function -- so most functions would start like this:

    sub myFunction {
        my ($self, $arg1, $arg2) = @_;
        ...
    }

Or, in older code, you might even see this:

    sub myFunction {
        my $self = shift; # implicitly get the first value from @_
        my $arg1 = shift;
        my $arg2 = shift;
        ...
    }

Notably, this didn't even check the number of arguments you passed. If you passed too many or too few arguments to a function which worked this way, the extra arguments would silently be ignored, or missing arguments would silently show up as undef.

Function signatures turn this into something that'll look much more familiar to users of other languages:

    sub myFunction ($self, $arg1, $arg2) {
        ...
    }

which includes checks to make sure that all the expected arguments (and no more) were passed.