perl -e"$_='1234567890';print 'x' x length substr($_,0,-4), substr($_,-4)"

##</code><code>##

#!/usr/bin/perl -w
use strict;
$|++;


my $string = '1234567890';
my $new_string = ( 'x' x length substr($string, 0, -4) ) . substr($string, -4);
print $new_string;