Can you have the script chdir $UploadDir prior to issuing the shell out to tar? Then omit the repeated use of $UploadDir from the command:

my $cmd = 'tar -cvf results.tar en_ran.out...';
chdir $UploadDir;
my $result = `$cmd`;
[download]

Otherwise, you'll need to closely read the man page for your tar command to see if there is a flag in your version to strip path elements either on the way into or out of the tar. GNU tar (in some versions) has --strip-components NUMBER, others have --strip-path NUMBER, these can remove path elements during extraction.

Phil