Maybe this will help...
## 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";
y is not defined
y is is defined now as 32
k is 3842 ...hey I'm defined now!
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!
I got off track, but since I'm talking about 0E0, I will show
some code for your amusement.
$|=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
# 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";
print "bogus_zero plus one is: $bogus_zero\n";
# "works" albeit with a warning
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