#!/usr/bin/perl -w
use strict;

my @urls = ('tp://www.foo.com/' , '://www.foo.com', 
            'http//:www.foo.com', 'www.foo.com'); 

foreach (@urls)
{
   s/^.*?www/www/;
   print "http://$_\n";
}

__END__
prints:
http://www.foo.com/
http://www.foo.com
http://www.foo.com
http://www.foo.com