# Solution 1
my $ref;
f(\$ref); # Just don't use $ref afterwards

# Solution 2
f(do{\my$x}); # Not really anonymous, since we have to
              # "invent" a variable name, but at least 
              # blessed by Larry

# Solution 3
f(\[]->[0]);  # Anonymous, but not elegant

# Solution 4
sub NULLREF { \ my $x }
f(NULLREF);# Maybe nice if we use this feature often in our
           # program and use NULLREF consistently, but still
           # not really anonymous