#!/usr/bin/perl
use warnings;
use strict;

my @pi = split //, '1415926535';
while (<DATA>) {
    1 while s/ ([^ ]*?)\Kx/$pi[length $1]/;
    print;
}

__DATA__
/bin/grep .......... /path/to/some/file
/bin/grep xxxxxxxxxx /path/to/some/file
/bin/grep x....x...x /path/to/some/file