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

Here's a quick and easy way to produce most of them:

use 5.010;
say 'SCALAR:  ', ref \undef;
say 'ARRAY:   ', ref [1..5];
say 'HASH:    ', ref { key => 'value' };
say 'CODE:    ', ref sub {};
say 'REF:     ', ref \\undef;
say 'GLOB:    ', ref \*_;
say 'LVALUE:  ', ref \substr "abc", 1, 2;
say 'LVALUE:  ', ref \vec 42, 1, 2;
say 'FORMAT:  ', ref *STDOUT{FORMAT}; # needs declaration below
say 'IO:      ', ref *STDIN{IO};   # actually prints IO::Handle
say 'VSTRING: ', ref \v5.10.0;
say 'Regexp:  ', ref qr/./;

format =
.

REF is just a reference to another reference. LVALUE is a special case of a scalar that has an external influence if it is modified.

IO is the base type behind the handles, you can make it appear explicitely using Acme::Damn from CPAN. As noted by Michael Carman in the comments, you really shouldn't be unblessing objects — don't use in real code.

use Acme::Damn;
say 'IO:      ', ref damn *STDIN{IO}; # really prints IO

The source for the ref function also has bits of code to display "BIND" and "UNKNOWN", but there shouldn't be a way to get those without messing with the internals. Blead also has an interesting unblessed "REGEXP" (different from the "Regexp" above); if someone knows how to make ref yield that...