Programming Perl, 3rd Edition

Using Hard References

Just as there are numerous ways to create references, there are also several ways to use, or dereference, a reference. There is just one overriding principle: Perl does no implicit referencing or dereferencing.^[4] When a scalar is holding a reference, it always behaves like a simple scalar. It doesn't magically start being an array or hash or subroutine; you have to tell it explicitly to do so, by dereferencing it.

Using a Variable as a Variable Name

When you encounter a scalar like $foo, you should be thinking "the scalar value of foo." That is, there's a foo entry in the symbol table, and the $ funny character is a way of looking at whatever scalar value might be inside. If what's inside is a reference, you can look inside that (dereferencing $foo) by prepending another funny character. Or looking at it the other way around, you can replace the literal foo in $foo with a scalar variable that points to the actual referent. This is true of any variable type, so not only is $$foo the scalar value of whatever $foo refers to, but @$bar is the array value of whatever $bar refers to, %$glarch is the hash value of whatever $glarch refers to, and so on. The upshot is that you can put an extra funny character on the front of any simple scalar variable to dereference it:

$foo         = "three humps";
$scalarref   = \$foo;         # $scalarref is now a reference to $foo
$camel_model = $$scalarref;   # $camel_model is now "three humps"

Here are some other dereferences:

$bar = $$scalarref; ...