use v5.14;
my $pattern = qr/#.{10}#.{13}#\./ms;
$_ = <<STRING;
#..#.....#.
..#...##...
.#....#..##
#..#....#..
..#...#..#.
.......#.#.
...........

STRING

say pos while /(?=$pattern)/g;
__DATA__
14
19
25

##</code><code>##

use v5.14;

my $length = 20; # Must be higher than the length of the longest line

# Add padding so that each line is $length chars long
# Without checking what char there are (ie, the \n are kept)
# So we know that if a char A is in position $a, the char B below it will be in position
# $a+$length, or there will be A.{$length-1}B 
$_ = pack "(A$length)*", <DATA>;

# $length - 2 is one line down, one char to the left
my $l1 = $length-2;

# $length + 1 is one line down, one char to the right
my $l2 = $length+1;

my $pattern = qr/#.{$l1}#.{$l2}#\./s;

# The first row and column have index 0 with this code
say join ", ", 1+int(pos() / $length), 1+(pos() % $length)  while /(?=$pattern)/g;
__DATA__
#..#.....#
..#...##
.#....#..##
#..#....#.....#
..#...#..#.##
.......#.#
...........

##</code><code>##

1, 2
1, 7
2, 1