while (@lines) {
	$output .= "<TR>";
	for ( 1 .. 3 ) {
		$output .= "<TD>" . (shift @lines) . "</TD>";
	}
	$output .= "</TR>";
}

##</code><code>##

my $fill = scalar @lines % 3;
while ($fill-- > 0) {
    push @lines, " ";
}