How do I use boolean variables in Perl?

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.

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.

Beautiful explanation given by bobf for Boolean values : True or False? A Quick Reference Guide

Truth tests for different values

                       Result of the expression when $var is:

Expression          | 1      | '0.0'  | a string | 0     | empty str | undef
--------------------+--------+--------+----------+-------+-----------+-------
if( $var )          | true   | true   | true     | false | false     | false
if( defined $var )  | true   | true   | true     | true  | true      | false
if( $var eq '' )    | false  | false  | false    | false | true      | true
if( $var == 0 )     | false  | true   | true     | true  | true      | true

Booleans in Raku (the programming language formerly known as Perl_6):

~$ raku
Welcome to ™ v2021.06.
Implementing the ™ programming language v6.d.
Built on MoarVM version 2021.06.

To exit type 'exit' or '^D'

> my $var = False;
False
> say $var;
False

> say $var.^name
Bool
> say $var.WHAT
(Bool)

> say ++$var
True
> say --$var
False

> say $var.Int
0
> say $var.Int + 1
1
> say ($var.Int + 1).Bool
True
> say $var.Int - 1
-1
> say ($var.Int - 1).Bool
True

> say $var.succ
True
> say $var.Int.succ
1
> say $var.pred
False
> say $var.Int.pred
-1

> say ++($var.Int); #ERROR
Cannot resolve caller prefix:<++>(Int:D); the following candidates
match the type but require mutable arguments:
    (Mu:D $a is rw)
    (Int:D $a is rw --> Int:D)
    #<SNIP>    
> say --($var.Int); #ERROR
Cannot resolve caller prefix:<-->(Int:D); the following candidates
match the type but require mutable arguments:
    (Mu:D $a is rw)
    (Int:D $a is rw --> Int:D)
    #<SNIP>

> exit

https://docs.raku.org/type/Bool
https://docs.raku.org/language/syntax#index-entry-Boolean_(literals)