The word you're looking for is "autovivification".

You can avoid autovivifying hash and array slots by checking they exist first. Example:

#!/usr/bin/perl

my %a;
if (exists $a{'abd'}
    and exists $a{'abd'}->[0]
    and defined $a{'abd'}->[0])
{
    print "yes \n";
}

use Data::Dumper;
print Dumper \%a;

[download]

An alternative way of doing it would be to use the autovivification module (requires Perl >= 5.8.3). This produces nicer looking code, at the cost of a non-core dependency.

#!/usr/bin/perl

no autovivification;

my %a;
if (defined $a{'abd'}->[0])
{
    print "yes \n";
}

use Data::Dumper;
print Dumper \%a;

[download]

In reply to Re: restricting values to a nested datastructure by tobyink
in thread restricting values to a nested datastructure by babp

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