Perhaps you might like to consider adding a portion of this post to the true false tutorial. This was posted in reply to a question on undef/false. At the very least please include undef as a false value not just 0 and ''.

You seem unclear between the difference between defined and false. This code should show you the difference.

In perl falseness includes these three miscreants only: 0 "" and undef. All else is true

UPDATE: Chipmunk did not like my evals so I have used 5 different perl idioms for if (condition){print this}else{print that}. Choices, choices. TIMTOWTDI TIMTOWODI. To avoid confusion these idioms all do exactly the same thing - test a condition and print someting if it is true and something different if it is not.

tachyon 

my $a; print "my \$a;\n";
print "Undefined\n" unless defined $a;
print "False\n" unless $a;

$a=0; print "\nmy \$a=0;\n";
if (defined $a) {
    print "Defined\n";
} else {
    print "Undefined\n";
}
if ($a) {
    print "True\n";
} else {
    print "False\n";
}

$a=''; print "\nmy \$a='';\n";
print eval'(defined $a) ? "Defined\n" : "Undefined\n"';
print eval'($a) ? "True\n" : "False\n"';

$a=1; print "\nmy \$a=1;\n";
print ((defined $a) ? "Defined\n" : "Undefined\n");
print (($a) ? "True\n" : "False\n");

$a='foo';print "\nmy \$a='foo';\n";
print '',(defined $a) ? "Defined\n" : "Undefined\n";
print '',($a) ? "True\n" : "False\n";

undef $a; print "\nundef \$a;\n";
(defined $a) ? print "Defined\n" : print "Undefined\n";
($a) ? print "True\n" : print "False\n";

[download]

This is about the only printing idiom that does not work! Sadly it is also the most elegant to my eyes. 

print (defined $a) ? "Defined\n" : "Undefined\n";

[download]

This only works with a null string and comma after the print as shown above print '', (cond)? foo : bar

In reply to Tutorial::What is true and false? by tachyon

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