Why do you need to simplify the code? Reducing the number of lines is not easy, but refactoring the similar parts to subroutines is possible:

sub assign_max {
    my ($x, $y) = @_;
    $s[$b1] = max($x, $y);
}

sub compare {
    my ($ar, $idx, $subst) = @_;
    if ($ar->[$idx] == 0 && $subst > 0) {
        $ar->[$idx] = $subst;
    }
}


$h = $s[$b] == 0 ? 0 : $s[$b] - 1 unless $l == $k;
$c = (0, 1, 2)[$l <=> $k];

if ($l < $k) {
    compare(\@s_rev, $d + 1, $k - 2);
    assign_max($k, $lt);
    $j++;

} elsif ($l > $k) {
    compare(\@s, $b + 1, $l - 2);
    assign_max($l, $kt);
    $i++;

} else {
    compare(\@s, $b + 1, $k - 2);
    assign_max($k, $lt);
    $h = 0;
}
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As you can see, assign_max uses @s and $b1 as globals.