Your question is not very clear to me, but, contrary to others who suggested to use references, I understand that you do NOT want the content of the hash to change when you change some variables in the environment. If you do this:

$test = 'test';
%hash = ( t => $test);
$test = 'changed';
[download]

the content of the hash does not change, it is still 'test', because the assignment:

%hash = ( t => $test);
[download]

is storing into the hash the content of the $test variable at the time this assignment is done. Modifying the $test variable afterwards will not modify the content of the hash.

But if you do:

$change = 'changed';
$hash{t} = $change;
[download]

then, of course the value in the hash will be modified because that's precisely a common way to modify the content of a hash for a given key.