Worst suvat calculator

$s = 0;
$u = 0;
$v = 0;
$a = 0;
$t = 0;
$unknown = 0;

get_input();
get_unknowns();
calculate();

sub get_input
{
print("What distance are you travelling? Input the value or 'u' for unknown.\n");
$s = <STDIN>;
chomp($s);

print("What is the initial velocity? Input the value or 'u' for unknown.\n");
$u = <STDIN>;
chomp($u);

print("What is the final velocity? Input the value or 'u' for unknown.\n");
$v = <STDIN>;
chomp($v);

print("What is the acceleration? Input the value or 'u' for unknown.\n");
$a = <STDIN>;
chomp($a);

print("How long is this journey? Input the value or 'u' for unknown.\n");
$t = <STDIN>;
chomp($t);
}

sub get_unknowns
{
    if($s eq "u"){
        $unknown = "s";
    } elsif($u eq "u"){
        $unknown = "u";
    } elsif($v eq "u"){
        $unknown = "v";
    } elsif($a eq "u"){
        $unknown = "a";
    } else {
        $unknown = "t";
    }
}

sub calculate
{
    if($unknown eq "s"){
        calculate_displacement($u, $v, $a, $t);
    } elsif($unknown eq "u") {
        calculate_initial_velocity($s, $v, $a, $t);
    } elsif($unknown eq "v") {
        calculate_final_velocity($s, $u, $a, $t);
    } elsif($unknown eq "a") {
        calculate_acceleration($s, $u, $v, $t);
    } else {
        calculate_time($s, $u, $v, $a);
    }
}

sub calculate_displacement
{
    my($initial_v, $final_v, $acceleration, $time) = @_;
    my $displacement = $time * $initial_v + 1/2 * $acceleration * $time * $time;
    print("The displacement of this journey is: $displacement m\n");
}

sub calculate_initial_velocity
{
    my($displacement, $final_v, $acceleration, $time) = @_;
    my $initial_v = sqrt($final_v * $final_v - 2 * $acceleration * $displacement);
    print("The initial velocity of this journey is: $initial_v metres per second.\n");
}

sub calculate_final_velocity
{
    my($displacement, $initial_v, $acceleration, $time) = @_;
    my $final_v = sqrt($initial_v * $initial_v + 2 * $acceleration * $displacement);
    print("The final velocity of this journey is: $final_v metres per second.\n");
}

sub calculate_acceleration
{
    my($displacement, $initial_v, $final_v, $time) = @_;
    my $acceleration = ($final_v * $final_v - $initial_v * $initial_v)/(2 * $displacement);
    print("The acceleration of this journey is: $acceleration metres per second per second.\n");
}

sub calculate_time
{
    my($displacement, $initial_v, $final_v, $acceleration) = @_;
    my $time = ($final_v - $initial_v)/$acceleration;
    print("The time taken for this journey to be completed is: $time seconds\n");
}