How do I use boolean variables in Perl?

Question

In Perl, the following evaluate to false in conditionals:

0
'0'
undef
''  # Empty scalar
()  # Empty list
('')

The rest are true. There are no barewords for true or false.

The most complete, concise definition of false I've come across is:

Anything that stringifies to the empty string or the string 0 is false. Everything else is true.

Therefore, the following values are false:

• The empty string
• Numerical value zero
• An undefined value
• An object with an overloaded boolean operator that evaluates one of the above.
• A magical variable that evaluates to one of the above on fetch.

Keep in mind that an empty list literal evaluates to an undefined value in scalar context, so it evaluates to something false.

---

A note on "true zeroes"

While numbers that stringify to 0 are false, strings that numify to zero aren't necessarily. The only false strings are 0 and the empty string. Any other string, even if it numifies to zero, is true.

The following are strings that are true as a boolean and zero as a number:

• Without a warning:
  • "0.0"
  • "0E0"
  • "00"
  • "+0"
  • "-0"
  • " 0"
  • "0\n"
  • ".0"
  • "0."
  • "0 but true"
  • "\t00"
  • "\n0e1"
  • "+0.e-9"
• With a warning:
  • Any string for which Scalar::Util::looks_like_number returns false. (e.g. "abc")

Perl doesn't have a native boolean type, but you can use comparison of integers or strings in order to get the same behavior. Alan's example is a nice way of doing that using comparison of integers. Here's an example

my $boolean = 0;
if ( $boolean ) {
    print "$boolean evaluates to true\n";
} else {
    print "$boolean evaluates to false\n";
}

One thing that I've done in some of my programs is added the same behavior using a constant:

#!/usr/bin/perl

use strict;
use warnings;

use constant false => 0;
use constant true  => 1;

my $val1 = true;
my $val2 = false;

print $val1, " && ", $val2;
if ( $val1 && $val2 ) {
    print " evaluates to true.\n";
} else {
    print " evaluates to false.\n";
}

print $val1, " || ", $val2;
if ( $val1 || $val2 ) {
    print " evaluates to true.\n";
} else {
    print " evaluates to false.\n";
}

The lines marked in "use constant" define a constant named true that always evaluates to 1, and a constant named false that always evaluates by 0. Because of the way that constants are defined in Perl, the following lines of code fails as well:

true = 0;
true = false;

The error message should say something like "Can't modify constant in scalar assignment."

I saw that in one of the comments you asked about comparing strings. You should know that because Perl combines strings and numeric types in scalar variables, you have different syntax for comparing strings and numbers:

my $var1 = "5.0";
my $var2 = "5";

print "using operator eq\n";
if ( $var1 eq $var2 ) {
    print "$var1 and $var2 are equal!\n";
} else {
    print "$var1 and $var2 are not equal!\n";
}

print "using operator ==\n";
if ( $var1 == $var2 ) {
    print "$var1 and $var2 are equal!\n";
} else {
    print "$var1 and $var2 are not equal!\n";
}

The difference between these operators is a very common source of confusion in Perl.

I recommend use boolean;. You have to install the boolean module from cpan though.