I want to know how my given code is working for you?

Uhm, I cut-and-pasted your code, changed the filenames and ran it from the command line?

Else tell me how to pass a perl variable value to Unix command?

I don't understand. Your initial post starts with:

$a=`dd if=/dev/$cmd of=/dev/null bs=4096 count=2 > ddlog.txt 2>&1`;
[download]

which passes a variable to the command. But what has that to do with drop down lists, or printing to terminals?