Programming Perl, 4th Edition

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, known as a sigil, 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 sigil. 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 sigil 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;

push(@$arrayref, $filename);
$$arrayref[0] = "January";            # Set the first element of @$arrayref
@$arrayref[4..6] = qw/May June July/; # Set several elements of @$arrayref

%$hashref = (KEY => "RING", BIRD => "SING");  # Initialize whole hash
$$hashref{KEY} = "VALUE";                     # Set one key/value pair
@$hashref{"KEY1","KEY2"} = ("VAL1","VAL2");   # Set two more pairs

&$coderef(1,2,3);

say $handleref "output";

This form of dereferencing can only make use of ...