Welcome to the Monastery! You have nice puzzle, where I think I would use a different approach. Basically, when a user comes up with a starting grid coordinate, and a word to insert (horizontally or vertically) at that point, all you want to do really is to determine the length of the user's proposed word, and do two things:

• check that the word does not extend past the edge of the grid, and
• extract just the list of current grid contents from the cells affected by the word, join those contents into a string, and -- given that empty cells are initialized with period characters -- see whether that string matches the proposed word.

# Assume this is a subroutine, called with these args:

my ($grid_size);  # number of rows, columns (square grid, right?)
my ($bgnX,$bgnY); # user's start coords;
my ($new_word);   # user's proposed word;
my ($direction);  # down or across

my $newlen = length( $new_word );
if ( $direction eq 'down' ) {
  if ( $bgnY + $newlen > $grid_size ) {
     return "$new_word doesn't fit going down from $bgnY\n";
  }
  my $ongrid = "";
  for ($bgnY .. $grid_size-1) {
     $ongrid .= $board[$bgnX][$_];
  }
  return "$new_word doen't match existing tiles\n"
      unless ( $new_word =~ /^$ongrid$/ );
}
else {
  # left as an exercise...
}
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