How can I get Perl's ref() function to return REF, IO, and LVALUE?

Question

The documentation for ref() mentions several possible return values. I understand most of them, but not REF, IO, and LVALUE. How would I write Perl code to cause ref to return those values?

After reading the documentation on typeglobs and file handles, I came close for IO with this code:

open(INPUT, '<', 'foo.pl');
print ref(*INPUT{IO}), "\n";  # Prints IO::Handle

For REF and LVALUE I tried several bizarre constructions, but had no success.

Answer 1

1. LVALUE

   perl5.8 -e '{$a = "aaa"; $b = \substr($a, 0, 2); print "$$b\n"; print ref($b)."\n"}'

   aa

   LVALUE

   This one is explained in perldoc -f ref

2. REF

   perl5.8 -e '{$a = "aaa"; $b = \\$a; print ref($b)."\n"}'

   REF

   It's basically a reference to a value that is itself a reference. Probably would have been better to do $b = \$a; $c = \$b; print ref($c)

Answer 2

REF means that you have a reference to a reference:

my ($x, $y, $z);
$x = 1;
$y = \$x;
$z = \$y;
print ref $z;  # REF

LVALUE refs are rare, but you can get them from certain functions that return lvalues. substr is one:

$x = 'abcdefg';
$y = \ substr($x, 0, 3);
print ref $y;  # LVALUE

IO::Handle objects are actually blessed IO refs:

$x = *STDIN{IO};
print ref $x;  # IO::Handle
print $x;      # IO::Handle=IO(0x12345678)

I'm not sure how to get an IO ref directly.