can you run the same commands using ssh from the command line?

$ ssh host umount -f /mnt/nfs_share_CrpgC
$ ssh host rm -rf /mnt/nfs_share_CrpgC
[download]

Also, Net::SSH::Perl is not being maintained anymore and it has lots of unsolved bugs in its CPAN RT queue. You should consider using Net::SSH2, Net::OpenSSH or Net::SSH::Expect instead.