In Perl, arguments are normally passed by reference. This means that if you modify a member of the array @_, you are actually modifying the original value you passed to the function.

sub foo { $_[0] ++ }
$a = 25;
foo($a);
print "$a\n";

This prints 26. Perl subroutines are often written this way:

sub foo {
    my ($arg) = @_;
    $arg ++
}

This copies the parameters into lexical (scope-local) variables. In this case, $a won't be incremented—only the lexical variable $arg will. For people coming from a C background this is more intuitive, emulating the pass-by-value practice of most programming languages.

In my parser, the first form is used so that we can maintain the regex position across function calls. It also avoids copying the string over and over. I recommend using the second form most of the time, but remembering the first form for when the need arises.

For more information, see the perlsub documentation.