undef is false by definition in Perl and so is absolutely fine whenever in a boolean context.

Autovivification only happens when required, such as during an assignment (but not for a test). However if a chain of references is required all the intermediate references will autovivify, but not the last item in the chain. Consider:

#!/usr/bin/perl
use strict;
use warnings;

my $x;
my $y;

if ($y) {  }
if (! $y) {  }
if ($y && ! $y) {  }
if (exists $x->{notdef}) {  }
if (defined $x->{notdef}) {  }
if ($x->{notdef}) {  }

print ref $x, "\n";
print "\$y is undef\n" if ! defined $y;

if (exists $x->{notdef}{notdeftoo}) {}

print "Now \$x->{notdef} exists\n" if exists $x->{notdef};

[download]

Prints:

HASH
$y is undef
Now $x->{notdef} exists

[download]

Note though that $x became a hash ref because it was treated as one even before any autovification took place which makes the dereference ok - $x is no longer undef.

True laziness is hard work

In reply to Re: When DOESN'T "Use of uninitialized value" show up? by GrandFather
in thread When DOESN'T "Use of uninitialized value" show up? by Tim McDaniel

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