Programming Perl, 4th Edition

Using a block As a Variable Name

Not only can you dereference a simple variable name, you can also dereference the contents of a BLOCK. Anywhere you’d put an alphanumeric identifier as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct type. In other words, the earlier examples could all be disambiguated like this:

$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
@{$arrayref}[4..6] = qw/May June July/;
${$hashref}{"KEY"} = "VALUE";
@{$hashref}{"KEY1","KEY2"} = ("VAL1","VAL2");
&{$coderef}(1,2,3);

not to mention:

$refrefref = \\\"howdy";
print ${${${$refrefref}}};

Admittedly, it’s silly to use the braces in these simple cases, but the BLOCK can contain any arbitrary expression. In particular, it can contain subscripted expressions. In the following example, $dispatch{$index} is assumed to contain a reference to a subroutine (sometimes called a “coderef”). The example invokes the subroutine with three arguments:

&{ $dispatch{$index} }(1, 2, 3);

Here, the BLOCK is necessary. Without that outer pair of braces, Perl would have treated $dispatch as the coderef instead of $dispatch{$index}.