#!/usr/bin/perl -w

use strict;

while(<>) {
         s/(.*?)(....)$/("X" x length($1)).$2/e;
         print;
}

##</code><code>##

#!/usr/bin/perl -w

use strict;

while(<>) {
    chomp;
    if(length > 4) {
         my $tail = substr($_,-4);
         $_=('X'x (length($_)-4)).$tail;
    }
    print "$_\n";
}