For many years Perl subroutines didn't even have method signatures so you had to unroll @ on the first line of every sub. I think even today it may still be considered experimental. Jeez!
The reason for the @_ is that it is a simple way to get both pass by value and pass by reference semantics.
The values in @_ are aliased to the values in the subroutine call.
Most of the time, you want pass by value semantics, so you unpack the array and copy the values into function variables. If you want to modify an argument, it's typically passed in as a reference, and you can mess with it that way.
However, there are times when it would be horribly inefficient to make those copies, or when you need to do some magic (generally best avoided in 99.999% of your code), that this makes possible.
Also, since Perl functions are always variadic, it means that it's easy to work with a variable list of arguments in a function. For example, if you are expecting a list of key value pairs for your function, you can simply write:
my %args = @_
Making signatures default will be a big improvement, but the power of the simple abstraction @_ provides should not be underestimated. It's actually an elegant solution to a complex problem.
