Without totally re-engineering nmap in Perl I can't think of a better way of doing it than:

#!/usr/bin/perl -w

use strict;

my @hosts = qw(localhost);

HOST:
foreach my $host (@hosts )
{
   open(NMAP, "nmap -O $host |") || next;

   while(<NMAP>)
   {
      chomp;
      if(/Remote operating system guess: (.*)/)
      {
         print "$host $1\n";
         next HOST;
      }
   }
}
[download]

You will need to run this as root and update the value of @hosts appropriately.

/J\