Programming Perl, 4th Edition

Symbol Table References

In unusual circumstances, you might not know what type of reference you need when your program is written. A reference can be created by using a special syntax, affectionately known as the *foo{THING} syntax. *foo{THING} returns a reference to the THING slot in *foo, which is the symbol table entry holding the values of $foo, @foo, %foo, and friends.

$scalarref = *foo{SCALAR};   # Same as \$foo
$arrayref  = *ARGV{ARRAY};   # Same as \@ARGV
$hashref   = *ENV{HASH};     # Same as \%ENV
$coderef   = *handler{CODE}; # Same as \&handler
$globref   = *foo{GLOB};     # Same as \*foo
$ioref     = *STDIN{IO};     # Er...
$formatref = *foo{FORMAT};   # More er...

All of these are self-explanatory except for the last two. *foo{FORMAT} is how to get at the object that was declared using the format statement. There isn’t much you can do with one of those that’s very interesting.

On the other hand, *STDIN{IO} yields the actual internal IO::Handle object that the typeglob contains; that is, the part of the typeglob that the various I/O functions are actually interested in. For compatibility with old versions of Perl, *foo{FILEHANDLE} was once a synonym for the hipper *foo{IO} notation, but that use is now deprecated.

In theory, you can use a *HANDLE{IO} anywhere you’d use a *HANDLE or a \*HANDLE, such as for passing handles into or out of subroutines, or storing them in larger data structures. (In practice, there are still some wrinkles to be ironed out.) The advantage of them is that they access only ...