#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

sub read_hosts {
    my $file = "/etc/hosts";
    open (my $fh, "<", $file) 
	or die "Can't open $file for read: $!";
    my @hosts = <$fh>;
    close $fh 
	or die "Cannot close $file: $!";
    return @hosts;
}

sub write_hosts {
    my $file = "/etc/hosts";
    open (my $fh, ">", $file) 
	or die "Can't open $file for read: $!";
    for my $host (@_) { 
	print $fh $host; 
    }
    # Or print $fh $_ for (@_);
    close $fh 
	or die "Cannot close $file: $!";
    return;
}

my @hosts = read_hosts;
print Dumper \@hosts;
$hosts[2] = "Test_host\t127.0.0.4\n";
write_hosts(@hosts);
my @updates = read_hosts();
print Dumper \@updates;

__END__

sudo perl hosts.pl
$VAR1 = [
          '127.0.0.1	localhost
',
          '127.0.1.1	user
',
          '# The following lines are desirable for IPv6 capable hosts
',
          '::1     ip6-localhost ip6-loopback
',
          'fe00::0 ip6-localnet
',
          'ff00::0 ip6-mcastprefix
',
          'ff02::1 ip6-allnodes
',
          'ff02::2 ip6-allrouters'
        ];
$VAR1 = [
          '127.0.0.1	localhost
',
          '127.0.1.1	user
',
          'Test_host	127.0.0.4
',
          '::1     ip6-localhost ip6-loopback
',
          'fe00::0 ip6-localnet
',
          'ff00::0 ip6-mcastprefix
',
          'ff02::1 ip6-allnodes
',
          'ff02::2 ip6-allrouters'
        ];