I believe Perl uses what's called a 'pass by reference' method to give variables to a function (only important to remember the name in other languages, e.g., C/C++, where you have a choice of by value or by reference). Anyway, what this means is that you can actually modify the variable inside the function without doing anything special. E.g.:

sub delete_first_char {
    $_[0] =~ s/^.//
}
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That code would delete the first character of a string (admittingly not the best way to do it). However, most subroutines you see do this:

sub whatever {
    ($var1,$var2,$etc) = @_;
...
}
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They do this so that they don't mistakingly alter/change the variable that was passed to them. Usually this is a good coding practice, however, quite certainly situations do arise where you wish to modify the variable that you pass in, in which case you just refer to it directly; i.e., $_[--elm # here--].