use strict;
use warnings;
use 5.012;

sub do_stuff(\$) {
    my $sref = shift;
    say $$sref;
}

##</code><code>##

my $str = 'hello';  
do_stuff($str);   #legal: no slash in position 1 of arg list

--output:--
hello

##</code><code>##

my $str = 'hello';
do_stuff(\$str);   #illegal: slash in position 1 of arg list

--output:--
Type of arg 1 to main::do_stuff must be scalar (not single ref constructor)

##</code><code>##

my $str = \'hello';
do_stuff($str);  #legal: no slash in position 1 of arg list

--output:--
SCALAR(0x100839858)