use strict;
use warnings; # ;-)
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Please note that $hash and %hash are different variables in perl. If you want to access hash element A with hash reference $hash, you would use $hash->{A}. See perlref for details.

Update: fixed a typo.

It may make it a little clearer if you rename some of the variables. For the first version if you rename the global version of hash to ghash you get:

use strict;
use warnings;

my (%ghash)=('A'=>1, 'B'=>2, 'C'=> 3);

rutine(\%ghash);

sub rutine
{    
    my ($hash)=@_;
    print "$hash{A} $hash{B} $hash{C}\n";    
}
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which generates a syntax error in the print - %hash is not known.

In the second case you have already seen that the syntax check results in an error message that says "... Global symbol "%hash" requires explicit package name at ...", which is Perl's way of saying "I don't know a variable".

The thing that is confusing here is that Perl uses the sigil (the $, @, % or *) at the start of the variable to tell what type of thing is returned by the variable. So when you write:

print "$hash{A} $hash{B} $hash{C}\n";

Perl knows that the variable is a hash (from the {A}) and that you want a scalar as a result (from the $ sigil). However, if the variable is a reference you have to dereference it first. The print statement in rutine needs to be dereferenced:

print "$hash->{A} $hash->{B} $hash->{C}\n";

For a little further reading about references see the Tutorials sectin, in particular intro to references and References quick reference.

Your second example does display the appropriate run-time error messages:

Global symbol "%hash" requires explicit package name at test-hashref.p
+l line 6.
Global symbol "%hash" requires explicit package name at test-hashref.p
+l line 6.
Global symbol "%hash" requires explicit package name at test-hashref.p
+l line 6.
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print "$hash->{A} $hash->{B} $hash->{C}\n";
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1 2 3
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"This is easily corrected by changing line 6 to:

print "$hash->{A} $hash->{B} $hash->{C}\n"; "
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I prefer:

print "$$hash{A} $$hash{B} $$hash{C}\n";
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You need to know them all. That means, practice.

Everybody needs to be able to read a log file and understand what was happening when your code was executed. Nobody is able to stay watching their scripts while they are working all the day long. So, the errors at the log files report how did it all work while you were not present. You also need to post your errors when you make a question here. Errors, explain a lot more than what our experience might know.

Everything that the errors might mean that you don't understand can be found at the perldocs. At this state of the perl language, it is very difficult to discover something that so many developers might have missed fixing, I guess.

But, I am sure that what would amaze you, like it does to me, is how many things can be achieved with this crazy perl language! Try all what the perldocs suggest, and you will see!