Sys::Hostname is in core, the others you'll have to install from CPAN ...

#!/usr/bin/perl
use strict;
use warnings;

use Sys::Hostname;
use Net::Address::IP::Local;
use Net::Netmask;

my $hostname = hostname;
my $ip       = Net::Address::IP::Local->public;
my $netmask  = Net::Netmask->new( $ip )->mask;

print "hostname : $hostname\n";
print "IP       : $ip\n";
print "Netmask  : $netmask\n";

__END__
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Output:

$ perl 1140691-2.pl
hostname : localhost
IP       : 10.5.50.27
Netmask  : 255.255.255.255
$
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Update: if you want the internal IP address you can use gethostbyname() and Socket, which is in Core, and dump Net::Address::IP::Local:

use Sys::Hostname;
use Socket;

my $hostname = hostname;
my $ip       = inet_ntoa( scalar gethostbyname( $hostname ) );
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See perlfaq9.