So, everything except arrays and hashes? Is "$foo" in "my $foo = 'hi'" a reference?

$foo is variable. If you do a

my $foo = Something.new();
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then the newly created object is stored by reference in the variable. It's the same for string literals, but since strings and immutable (and so-called "value types"), it doesn't make a difference for strings.

How would I create an alias (a reference) to a sub?

sub foo($x, $y) { say $x ~ $y }
my $foo_ref = &foo;   # leaves $foo_ref writable
my $foo    := &foo;   # basically the same, but you can't
                      # assign to $foo afterwards

# passing values to a signature is a form of 
# binding too, so this works:
sub c(&func) {
  func(42, 23);
}
c(&foo);
c($foo);
c($foo_ref);
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