#! /usr/bin/env perl
use strict;
use warnings;
use 5.020;


sub function {
        # arguments are aliased to @_
        # Overwrite the first argument
        $_[0] = "Hallo Welt";
}

my $foo = "Hello World";

say "Vorher: $foo";

function($foo);

say "Nachher: $foo";


__END__;

#! /usr/bin/env perl
use strict;
use warnings;
use 5.020;

# Prototype defines, we expect ONE argument and we pass it by reference
# (the user doesn't need to reference it herself)
sub function (\$) {
        my ( $ref ) = @_;

        $$ref = "Hallo Welt";
}

my $foo = "Hello World";
say "VORHER: $foo";

# see, no reference by user here (\$foo)!
function($foo);

say "NACHHER: $foo";



__END__;