sub foo
{
    my $s = shift;

    if (defined $s) {

        if ($s ne "") {
            #
            # 150 lines of code
            #
            }
        else {
            die "String is empty";
            }
        }
    else {
        die "String is null";
        }
    }

=>

sub foo
{
    my $s = shift;

    unless (defined $s) {
        die "String is null";
        }

    if ($s eq "") {
        die "String is empty";
        }

    #
    # 150 lines of code
    #
    }