Why does Perl autovivify in this case?

Question

Why does $a become an arrayref? I'm not pushing anything to it.

perl -MData::Dumper -e 'use strict; 1 for @$a; print Dumper $a'
$VAR1 = [];

Answer 1

$a becomes an ARRAY reference due to Perl's autovivification feature.

Answer 2

It is because the for loop treats contents of @$a as lvalues--something that you can assign to. Remember that for aliases the contents of the array to $_. It appears that the act of looking for aliasable contents in @$a, is sufficient to cause autovivification, even when there are no contents to alias.

This effect of aliasing is consistent, too. The following also lead to autovivification:

• map {stuff} @$a;
• grep {stuff} @$a;
• a_subroutine( @$a);

If you want to manage autovivification, you can use the eponymous pragma to effect lexical controls.

Answer 3

When you treat a scalar variable whose value is undef as any sort of reference, Perl makes the value the reference type you tried to use. In this case, $a has the value undef, and when you use @$a, it has to autovivify an array reference in $a so you can dereference it as an array reference.

Answer 4

$a and $b are special variables in Perl (used in sort) and have a special scope of their own.

perl -MData::Dumper -e 'use strict; 1 for @$c; print Dumper $c'

produces

Global symbol "$c" requires explicit package name at -e line 1.
Global symbol "$c" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.