Maybe this will help...

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

## The ||= operator tests for "truthfulness"
## The //= operator tests for "definedness"


my $z = 25;
my $x = 550;
$z ||= $x;
print "$z\n";  #prints 25 because z is already true

my $y;  # $y is undefined
print "y is not defined\n" if !defined $y;
$y //= 32;
print "y is is defined now as $y\n" if defined $y;

my $k;  #$k is undefined
        #An undefined value evaluates to "false"
        #and the assignment proceeds
        
$k ||= 3842;
print "k is $k ...hey I'm defined now!\n";


__END__
25
y is not defined
y is is defined now as 32
k is 3842 ...hey I'm defined now!
[download]

Update:
There is a rather strange thing that can happen in Perl.
It is possible for Perl to return a "true", "zero" value.
It does this by returning the string "0E0": 0 * 10**1 =0 numerically, but that evaluates to "true" in a logical sense. This is used in the Database Interface for example. You might get back an answer "hey I worked, but I didn't modify any rows!". In an language like C or Java, you have to have two variables: Number of rows and did it work or not? In Perl this can be expressed in a single value. "I worked, but didn't modify any rows!". A "0" is logical False while a 0E0 is logical True- pretty cool!

Update:
I got off track, but since I'm talking about 0E0, I will show some code for your amusement.

#!/usr/bin/perl -w
#use strict;
$|=1;  #Turn off STDOUT Buffering

#  This is wild but Perl has
#  a special string that will evaluate
#  to  a "true", "zero" value and can be used
#  in a numeric computation - even with
#  warnings!

#  This is so obscure that it must be
#  depreciated in favor of the 0E0 notation.
#  I personally wouldn't use this, and I
#  show it just for amusement. This just
#  Perl trivia.

my $str_zero = "0 but true"; ### special string ###
$str_zero += 1;
print "new str_zero is: $str_zero\n";
# No warning, this is the same as 0E0

my $bogus_zero = "0 bogus";
$bogus_zero +=1;
print "bogus_zero plus one is: $bogus_zero\n";
# "works" albeit with a warning

__END__
new str_zero is: 1
Argument "0 bogus" isn't numeric in addition 
   (+) at C:\TEMP\zeroButTrue.pl line 23.
bogus_zero plus one is: 1
[download]

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