use v5.12;
use warnings;
# https://perlmonks.org/?node_id=11141581

my $in = "anXwxdXdzd";
my $pi = "1415926535";

my @in = split //, $in;
my @pi = split //, $pi;

my $re = join "",
  map {$in[$_] eq "X" ? $pi[$_] : '.' }
  0..9;

$re = "/bin/grep $re /path/to/some/file";

say $re;