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

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.