SCALAR is a reference to a scalar; HASH is a reference to a hash; and so on (ARRAY, CODE, ...).

REF is a reference to any of those, e.g.

$ perl -wle 'use strict; my $x = 1; my $y = \$x; my $z = \$y; print "Y
+: ", ref($y), " Z: ", ref($z)'
Y: SCALAR Z: REF
$
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I vaguely recall using this feature some time ago when testing some deep copying of a complex data structure although I can't remember the exact details. The basic idea was to continually dereference to a scalar while ref() returned REF (e.g. $deref = $$ref); then to deference to the appropriate datatype (e.g. @array = @$ref if ref($ref) eq 'ARRAY').

I can't provide any help with the internals.