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;

[download]
OUTPUT: 
/bin/grep ..1...6... /path/to/some/file

[download]

edit

FWIW: Perl can apply mask operations with & , ^ and | , but this would also first require translating X to chr(255) and else to chr(0). see tr and s for that approach.

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

updates

In reply to Re: string substitution by LanX
in thread string substitution by Anonymous Monk

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