What is wrong?

Not much, I almost saw a stroke of genius in what you tried to do. The only problem for you is, that unix shells don't work like calculators.

What you want to archive can be done by using the following code:

my $value =$ENV{"one"} / $ENV{"two"};
print "my value is $value\n";
[download]

$ENV{x} allows you access to the value that is stored in the environment variable x.

/oliver/