system is executing the cd command in a forked process and returns to your Perl script. As it's happening in a different process, the working directory of your script doesn't change.

Instead of the system call, use chdir:

chdir '/home/sa/temp';
[download]

update -- add links to docs
update the second... i'm guessing the downvote was because of my erroneous description of what system was doing (as eff_i_g explains better). Fixed description to better reflect that. That'll teach me to read the docs better.