Why does Perl's autovivification work in this case?

autovivification, perl

Solution

Your code is missing

use strict;

C:\Temp> hui
Can't use string ("foo") as a HASH ref while "strict refs" in use at 
C:\Temp\hui.pl line 7.

Make sure all your scripts start with:

use strict;
use warnings;

Given:

$hash{hello} = "foo";

`$hash{hello}` is NOT a hash reference.

$hash{hello}{world} = "bar";

treats the string `"foo"` as a hash reference and creates the hash `%main::foo` and sets `$foo{world}` to `"bar"`.

When you do:

print Dumper \%hash;

it only prints the contents of `%hash`. Whereas, when you do

print $hash{hello}{world} . "\n";

it prints `$foo{world}`.

Without `strict`, you do not get to find out that the script has trampled all over the package name space.

Add a

print Dumper \%main::;

or

print Dumper \%main::foo;

to inspect the symbol table after you run this.

Problem

Can some one help me understand the output of this Perl program: ``` use Data::Dumper; my %hash; $hash{hello} = "foo"; $hash{hello}{world} = "bar"; print $hash{hello} . "\n"; print $hash{hello}{world} . "\n"; print Dumper(\%hash); ``` And the output: ``` foo bar $VAR1 = { 'hello' => 'foo' }; ``` Where is the "foo" coming from? How come it isn't printed out by dumper? Note that if I swap the order of the assignments: ``` use Data::Dumper; my %hash; $hash{hello}{world} = "bar"; $hash{hello} = "foo"; print $hash{hello} . "\n"; print $hash{hello}{world} . "\n"; print Dumper(\%hash); ``` my output is what I expect: ``` foo $VAR1 = { 'hello' => 'foo' }; ``` EDIT: I know that `use strict;` would catch this, but I'm more interested in know how the string "foo" is still being printed.

Original source