I think user would expect single mouse click, not only motion, should produce a paint stroke. Plus, not nice that with pointer just above grid line, an item from row below is changed (do bounding boxes of characters overlap slightly, vertically?).

With mouse button pressed, "Motion" event targets same "current" item, but X-Y coordinates are updated continuously. In fact, knowing these X-Y, we can ignore what's "current", fix all of the above and write it simpler:

# ...
$canvas-> Tk::bind( '<B1-Motion>', [ \&set_coord, Ev('x'), Ev('y') ]);
$canvas-> Tk::bind( '<Button-1>', [ \&set_coord, Ev('x'), Ev('y') ]);
$canvas-> Tk::bind( '<Enter>', [ configure => '-cursor', 'crosshair' ]
+);
# ...

sub set_coord {
    my ( $canv, $x, $y ) = @_;
    
    my $r = int( $y / $tile_h );
    my $c = int( $x / $tile_w );
    my ( $item ) = $canv-> find( withtag => "$r-$c", );
    $canv-> itemconfigure( $item, -text => $default_char );
}
[download]

Further nitpicks :) : the $default_char = ' '; appears to belong to initialization section, but does nothing. Could be moved somewhere further. + Some would argue that widgets' tab-order (i.e. I press Tab and focus moves from left to right instead of ...in unexpected ways) is important.