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perlreferenceautovivification

Why does Perl autovivify in this case?

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 = [];
Solution

  • 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.