Though I don't know exact why you can't use DBI, a read through Yes, even you can use CPAN might be useful for you.

You say you can accomplish your goal on the command line, but not with Perl. Perl is literally passing the string you are storing in $cmd to the OS. It should not be possible for that same exact string to fail in the Perl context and succeed on the command line (assuming you are performing the actions with the same environmental variables). What is the result when you modify your code to literally print the content of $cmd to STDOUT and then type that same code yourself?