The problem lies here:

if ($answer = "n")
{
&start();
}
elsif($answer = "y")
{
[download]

= is used for assignment. == and eq are used for equality testing of numbers and strings respectively. Also, you will want to make your code case insensitive.

The corrected code is:

$answer = lc($answer);
if ($answer eq "n")
{
&start();
}
elsif($answer eq "y")
{
[download]

Another alternative altogether would be to check $answer with a regex.