As mentioned by Anonymous Monk above, has an autovivification module for those (rare) occasions when you don’t want it to occur:

#! perl
use strict;
use warnings;

print "\nWITH autovivification:\n";
my $hashref1;
printf "1. \$hashref1 %s defined\n", (defined $hashref1 ? 'IS' : 'is N
+OT');
my $foo = $hashref1->{bar};
printf "2. \$hashref1 %s defined\n", (defined $hashref1 ? 'IS' : 'is N
+OT');

no autovivification;

print "\nWITHOUT autovivification:\n";
my $hashref2;
printf "3. \$hashref2 %s defined\n", (defined $hashref2 ? 'IS' : 'is N
+OT');
my $baz = $hashref2->{bar};
printf "4. \$hashref2 %s defined\n", (defined $hashref2 ? 'IS' : 'is N
+OT');
[download]

Output:

 0:43 >perl 830_SoPW.pl

WITH autovivification:
1. $hashref1 is NOT defined
2. $hashref1 IS defined

WITHOUT autovivification:
3. $hashref2 is NOT defined
4. $hashref2 is NOT defined

 0:43 >
[download]

This may (occasionally) be a handy tool to have in the toolbox.

Hope that helps,