Well, because  my %CS101 is not a global, its a lexical , lexicals don't have package names, ... what you have there is fake global

This is what you should have written

my %CS101 ...
ave(\%CS101);
sub ave {
    my( $cs ) = @_;
    print "$_\n" for values %$cs;
}
sub avee {
    my( $cs ) = @_;
    for my $k (keys %$cs ){
        print $cs->{$k}, "\n";
    }
}
[download]

Subroutines should take arguments, not work with global variables

You should stop using fake globsl , file scope my globals, lexical globals , global lexicals, you're nullifying the helpfulness of strict :) read about it http://perl.plover.com/flagbook/yak/Chi/slide040.html

Tutorials: Variable Scoping in Perl: the basics,
Coping with Scoping , Mini-Tutorial: Perl's Memory Management,
Lexical scoping like a fox,
Read this if you want to cut your development time in half!,
Closure on Closures , perlref#Circular References,
Memory leaks and circular references,
Circular references and Garbage collection.,
make perl release memory,
about memory management , Re^3: Scope of lexical variables in the main script , Re: What's the weaken here for?,
Devel::Cycle,
Devel::NYTProf,
Devel::Leak Devel::LeakTrace,
WeakRef

Big thanks, you are definitely right.

Howdy adriang, welcome back to the Monastery!

You're only declaring %CS101 after calling ave(), so at the time ave is running, it does not exist yet and therefore can't be accessed. Move the declaration to somewhere before the subroutine call, and all will be fine.

Thanks for the quick replay. Yes, this is true but the question is why I didn't get any error? Thanks in advance

Good question, actually: and my apologies, I misread your post.

Looking at the script again, I think that the reason is as follows. When Perl compiles your script, the fact that ave is called before %CS101 doesn't matter: by the time the Perl compiler gets to the body of ave, %CS101% has been declared and thus can be accessed.

However, when Perl runs your script, ave is called before %CS101 gets assigned a value, so the hash is empty and you don't get any output.