The values that evaluate to false in boolean context in Perl are undef, the number 0 (also when written as 0.00), the empty string, the empty array, the empty hash. In general anything that is considered to be empty. Everything else evaluates to true.

undef
0
0.00
''
'0'
my @arr;
my %h;

examples/boolean.pl

use strict;
use warnings;
use 5.010;

my $x;
my @arr;
my %hash;

if ($x) {
    say 'undef is true';
} else {
    say 'undef is false';
}

if (@arr) {
    say 'empty array is true';
} else {
    say 'empty array is false';
}

if (%hash) {
    say 'empty hash is true';
} else {
    say 'empty hash is false';
}

my @values = (0, 0.00, '', '0', '00', "0\n", [], {}, 'true', 'false');
for my $value (@values) {
    if ($value) {
        say "$value is true";
    } else {
        say "$value is false";
    }
}

The output will be:

undef is false
empty array is false
empty hash is false
0 is false
0 is false
 is false
0 is false
00 is true
0
 is true
ARRAY(0x55e25f1ce470) is true
HASH(0x55e25f1f1080) is true
true is true
false is true

(When printing "0\n" the "is true" part was printed on the next row because of the newline we print.)

Boolean context

Boolean context means an if statement, and unless statement. The conditional of a while loop, a ternary operator, etc.

undef and being defined

Being "defined" or being "undef" have different meaning in Perl.