#!/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
[download]

For each x, find the string between it and its preceding space. Its length is the position in the "mask".

Update: If the path can contain x, you need more caution, e.g. s/^[^ ]+ ([^ ]*?)\Kx/$pi[length $1]/g.