I think that you need to check two things:

1) Is your ssh connection working? perhaps you can put the command '@ar=$ssh->cmd("hostname")` in your script (instead of the existing @ar= command) and see if you get any print output.

2) Does the ssh login username and password, and the cd and cat commands (sent via $ssh->cmd) work if done manually?

I might do this another way (using @ar = `ssh user@hostname 'cat /var/lib/dhcp | grep $ARGV[0]'` and having ssh keys in place to avoid needing a password, as per the man ssh keygen and google).

Your way could/should work, might just need to troubleshoot a bit.