#!/usr/bin/perl -w
use strict;

my $a = undef;
print "a=$a\n";  #warning undef var !
$a ||= "";       # $a = $a || "";
print "a=$a\n";  #no warning "" null string

my $b= "0";      #gottcha with string of "0"
print "b=$b\n";
$b ||= "";
print "b=$b\n";  #sets b to "" (changes "0" to "")

my $c = "xyzzy";
$c ||= "";
print "c=$c\n";  #c still "xyzzy" !!

my $d = "";
$d ||= "default";
print "d=$d\n";  #d is "default"

my %hash;
my $e = "";
$hash{'key1'}{'key2'} = $e  ||= "--";
print "$hash{'key1'}{'key2'}\n"; #prints "--"
__END__
Output:
Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\scratch.pl line 5.
a=
a=
b=0
b=
c=xyzzy
d=default
--