#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %hash = (key1 => 1, key2 => 2, key3 => 3);
add_to_hash(\%hash);
die Dumper \%hash;

sub add_to_hash {
    my $h_ref = shift or die;
    $h_ref->{key4} = 4;
}