Maybe your $ENV{PATH} is messed up or you have aliased perl to /home/nitishb/perl5/perlbrew/perls/perl-5.16.3/bin/perl.

You will need to debug your shell setup and find out why invoking Perl as perl does not seem to use the correct Perl. Personally, I prefer to use absolute paths instead to choose the correct Perl. Maybe try:

curl -L http://cpanmin.us | /home/nitishb/perl5/perlbrew/perls/perl-5.
+16.3/bin/perl - App::cpanminus
[download]

Without seeing your environment, there is very little to be done otherwise.