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.

DWIM is Perl's answer to Gödel

In reply to Re: Reference curiosity? by GrandFather
in thread Reference curiosity? by tamaguchi

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