问题
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'
$VAR1 = [];
回答1:
It is because the for loop treats contents of @$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.
回答2:
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.
回答3:
$a becomes an ARRAY reference due to Perl's autovivification feature.
回答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.
来源：https://stackoverflow.com/questions/2206836/why-does-perl-autovivify-in-this-case