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A package is Perl's namespace. A module is a file (.pm) that defines a package.

# Counter.pm — a module file
package Counter;
use strict;
use warnings;

sub new {
    my ($class, %args) = @_;
    my $self = {
        count => $args{start} // 0,
    };
    bless $self, $class;
    return $self;
}

sub increment {
    my $self = shift;
    $self->{count}++;
}

sub get {
    my $self = shift;
    return $self->{count};
}

1;       # modules must return true

bless is what makes an object an object in Perl. It tags a reference with a class name. Method calls on a blessed reference look in that package.

Usage:

use Counter;

my $c = Counter->new(start => 100);
$c->increment;
$c->increment;
print $c->get, "\n";   # 102

Method dispatch — $obj->method(args) is sugar for Class::method($obj, args). The first arg is always the invocant.

Inheritance with @ISA:

package Counter::Bounded;
use parent 'Counter';        # alternative: our @ISA = ('Counter');

sub increment {
    my $self = shift;
    return if $self->{count} >= $self->{max};
    $self->SUPER::increment();   # call parent's version
}

Modern Perl (Moose, Moo) gives you classes with attributes, defaults, types, and roles. For Judge0's plain Perl 5.28, the manual bless style is what works.

For single-file scripts, you can define a package inline:

package Foo {
    sub new { ... }
}
my $f = Foo->new();