I opted for the "abuse regex" approach (because it's fun, and you can do it incrementally one s/// at a time until it works): 

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1121552

use strict;
use warnings;

for my $y ( 1..25 )
  {
  local $_ = tile(5,$y);
  /1$/ and !/-/ and print "success  5,$y\n";
  }

sub tile
  {
  my ($w, $h) = @_;
  my $wm1 = $w - 1;
  local $_ = ( '-' x $w . "\n" ) x $h;
  s/-/1/;
  1 while  # print("$_\n"),
    s/1..{$w}\K-/1/s ||
    s/1\n.*\K-(?=\n)/2/ ||
    s/1(?=.*$)\K-/4/ ||
    s/2.{$wm1}\K-/2/s ||
    s/2.{$wm1}\n\K-/1/ ||
    s/-(?=2.*$)/3/ ||
    s/-(?=..{$w}3)/3/s ||
    s/^.*\K-(?=3)/2/ ||
    s/-(?=.{$wm1}\n3)/4/ ||
    s/-(?=.{$wm1}4)/4/s ||
    s/^.*4\K-/1/ ||
    s/-(?=.{$w}4\n)/3/s;

  return $_;
  }

[download]

which prints 

success  5,1
success  5,3
success  5,7
success  5,9
success  5,11
success  5,13
success  5,17
success  5,19
success  5,21
success  5,23

[download]

Sure looks like multiple solutions exist.

In reply to Re: [OT (though it might involve some Perl programming)] One for the weekend. (Updated: significant typo!) by Anonymous Monk
in thread [OT] One for the weekend. (Updated: Answer posted!) by BrowserUk

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