Hello nysus,

Have you tried to execute the script with sudo?

Sample:

For me the following worked just fine:

#!/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'
        ];
[download]

Update: Adding print $fh $_ for (@_);

Update2: Or use Sudo module:

Make sure the script "hosts.pl" is executable chmod +x hosts.pl

#!/usr/bin/perl
use strict;
use warnings;
use Sudo;
my $su;

my $name = "root";
my $pass = "password";
$su = Sudo->new( { sudo         => '/usr/bin/sudo',
           #sudo_args    => '...',   
           username     => $name,
           password     => $pass,
           program      => '/home/user/hosts.pl',
           #program_args => '...'
         }
    );

my $result = $su->sudo_run();
if (exists($result->{error})) {
    printf "STDERR: %s\n",$result->{error};
    #&handle_error($result); 
}
else {
    printf "STDOUT: %s\n",$result->{stdout};
    printf "STDERR: %s\n",$result->{stderr};
    printf "return: %s\n",$result->{rc};
}

__END__

$ perl sudo.pl
STDOUT: $VAR1 = [
          '127.0.0.1    localhost
',
          '127.0.1.1    user
',
          '
',
          '::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'
        ];

STDERR: 
return: 1
[download]

Hope this helps.