References

$ref = \something            

ref = something

$hashref = {a=>1, 
            b=>2};   
$arrayref = [1, 2, 3, 4];  

$scalar = 'h';             
$scalarref = \$scalar;     

sub a {1};          
$subref = \&a;                   
# or $subref = sub {1};
&$subref;
a;

$refref = \$ref; 
                    
$undef;
$undefref = \$undef; 

hashref = {'a': 1,    #'dictionary'
           'b': 2}
arrayref = [1, 2, 3, 4]  #'list'
arrayref = (1, 2, 3)     #'tuple'
 
scalarref = 'h'

def a(): return 1
subref = a           #'function'

subref()  # parentheses trigger a subroutine
a()       

reflist = [subref]     # no exact match
reftuple = (subref,)  

undefref = None