Perl curio: Dereferencing blocks

We're all familiar with references and Use Rule 1:

You can always use an array reference, in curly braces, in place of the name of an array.

This leads to code like ${$foo} (dereference a scalar reference) or @{$bar{baz}} (dereference an array reference stored in a hash).

The curious part: The curly braces actually form a block, i.e. you can put multiple statements in there (just like do BLOCK), as long as the last one returns a reference:

% perl -E 'use strict; use warnings; ${say "hi"; \$_} = 42; say $_'

This block also gets its own scope:

% perl -E 'use strict; use warnings; ${my $x = "hi"; say $x; \$x} = 42; say $x'
Global symbol "$x" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.

$x isn't visible outside the ${ ... } block it was declared in.

% perl -E 'use strict; use warnings; ${my $x = "hi"; say $x; \$x} = 42;'

Leave a comment

About mauke

user-pic I blog about Perl.