jodv has asked for the wisdom of the Perl Monks concerning the following question:

So, I just want to do a hash where the value referred to by each key is a reference to another hash. 
#!/usr/bin/perl

use strict;
use warnings;

my %testhash;

$testhash{ "Tier 1" } = { "Tier 2" => "Tier 3" };

my $ref = $testhash{ "Tier 1" };

print $testhash{ "Tier 1" }, " is a reference.\n";

######  Works Great  ########
print %$ref, " is dereferenced.\n";    

######  Dies Horribly  ######
print %$testhash{ "Tier 1" }, " is dereferenced.\n";

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Since $ref is the exact same hash reference as $testhash{ "Tier 1" }, why does Perl spit out "Global symbol "$testhash" requires explicit package name" when I try to dereference the latter? I'm guessing Perl thinks that $testhash is a non-existent scalar, but I can't figure out why it isn't returning as the hash reference value and derefencing it. And to prove they are the same ref: 
print $ref, " is \$ref\n";
print $testhash{ "Tier 1" }, " is \$testhash\{ \"Tier 1\" \}\n";
------------------------------
HASH(0x8dc2bdc) is $ref
HASH(0x8dc2bdc) is $testhash{ "Tier 1" }

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Thanks!

Replies are listed 'Best First'.
Re: Nested Hash Dereferencing Syntax
by ikegami (Patriarch) on Feb 24, 2009 at 17:27 UTC
    Perl sees %$testhash, but doesn't know of any $testhash. You want %{ $testhash{ "Tier 1" } }
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      Nice. I tried parens, but not curlies, in my attempt to make sure Perl was evaluating the value of $testhash{ "Tier 1" } before trying to dereference.
      Do you know why curlies, and not parens, are necessary here? Just curious.
      Thanks for your help!
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        Here is an explanation of why parens cannot be used there.

        If you could write %($hash{$key}) then you should expect to be able to write %($hash{$key}){$other_key}. But perlvar tells us that $( is a built-in special variable, so correctly knowing how to parse that would be very, very hard.

        Incidentally %( is a valid Perl hash, though it is not used for anything and accessing it can get tricky. But the following is valid Perl: 

        %( = 0..9;
print keys(%();

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        Do you know why curlies, and not parens, are necessary here?

        Because parentheses only ever group things, never performing any nesting or scoping or accessing.

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        No, I don't know why. Curlies are always used where are variables name is expected. 
        my $x = ${$hash_ref}{key};  # Original syntax
my $x = $hash_ref->{key};   # Newer, cleaner syntax

print($foo, "bar\n");
print("${foo}bar\n");       # same
print("$foobar\n");         # XXX

my $x = ${"varname"};       # Symbolic reference

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        etc.

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        It strikes me that curlies (or braces) are used for legacy reasons given that the provenance of the language is the *NIX shell (see, to name but one, Perl).

        As we all know, *NIX shell uses braces in references to environment variables - when and only when the variable isn't followed by a printable, non-word boundary char can the braces be omitted e.g. ${foo}_bar is the var foo with _bar appended, whereas $foo_bar refers to a variable named foo_bar.

        This (string interpolation) is true for both shell and perl, but interestingly (I really ought to get out more:-) not in make(1) scripts - which can take braces or parens as separators.

        Update:

        Thanx to tilly for the hint.

        A user level that continues to overstate my experience :-))
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Re: Nested Hash Dereferencing Syntax
by planetscape (Chancellor) on Feb 24, 2009 at 18:52 UTC

    You may also wish to read through the Tutorials in this category: References.

    HTH,

    planetscape
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Re: Nested Hash Dereferencing Syntax
by toolic (Bishop) on Feb 24, 2009 at 17:36 UTC
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