perl -e"$_='1234567890';print 'x' x length substr($_,0,-4), substr($_,
+-4)"
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or, in full-program form:

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


my $string = '1234567890';
my $new_string = ( 'x' x length substr($string, 0, -4) ) . substr($str
+ing, -4);
print $new_string;
[download]

~Particle *accelerates*