What you're trying to do is use a "symbolic reference". Searching on that term will probably help a lot.

I see two problems here:

1. First, use strict will prevent you from using symbolic references at all, because they are generally a bad idea. You should probably do as toolic suggested, and use a hash of hashes. That said, to turn off strictness, use no strict 'refs'; within a block.
2. Second, symbolic references work by looking up the name of a variable in the symbol table. Only global variables live in the symbol table (though many are placed in packages); the other major type of variable in perl is a lexical variable, which does not appear in teh symbol table. You declare lexical variables using teh keyword my. By saying my %cs101 = ..., you've made it a lexical variable, gaurenteeing it will never show up in the symbol table, and a symbolic ref to it will not work. One way to declare global variables is to use the keyword our (this has aliasing affects in a package as well, but that won't affect things here.)

Here's a working version:

use strict;
use warnings;

our %cs101 = ("joe"=>    "80" );

print "Choose which hash\n";
chomp (my $input = <STDIN>);
#my $whichhash=\%cs101;
my $whichhash="$input";

sub hashvalues {
   no strict 'refs';
   while( my ($key, $value) = each (%{$whichhash}) ) {
       print "$key $value\n";
   }
}

&hashvalues;
[download]

Read perldoc perlref, focusing on the sections for symbolic references.

--Clinton