You can use Sys::HostIP (thanks for the tip ybiC which seems to work fine. If you don't want to use a module for one simple function, here's a light weight alternative that works out great for me.

Here's a program that demonstrates it in action.
$addr = $^O eq 'MSWin32' ? slurp_ip('ipconfig') : slurp_ip('/sbin/ifco
+nfig eth0');

sub slurp_ip {
  my $cmd = shift;
  my ($ip, $name);

  $name  = hostname();
  $ip  = join(".",unpack("C4",(gethostbyname($name))[4]));
  
  if (!$ip) { 
    $ip = $1 if `$cmd` =~ /(\d+\.\d+\.\d+\.\d+)/;
  } 
  
  return $ip;
}

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