If $package_name is just a top level package, this is easy:

First of all, the main symbol table %:: (aka %main::) is a hash; you want to look up the key named $package_name."::" (assuming $package_name doesn't already have the all-important trailing ::). This is just $::{$package_name."::"}.

Now, the values in a symbol table are globs (with the exception of declared-but-not-defined subs, which are scalars giving prototype information); you want to get the hash reference out of the glob. This is done with the *FOO{THING} syntax (enclosing our FOO in {} since its an expression that evaluates to a glob, not a simple glob name): *{$::{$package_name."::"}}{HASH}.

Voila:

use strict;
use Data::Dumper;
my $package_name = "Data";
print Dumper *{$::{$package_name."::"}}{HASH};
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gives:

$VAR1 = {
          'Dumper::' => *{'Data::Dumper::'}
        };
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You can then go ahead and use that hash ref to get a nested package, look up a glob in it, and extract a hash reference from it:

use strict;
use Data::Dumper;
my $top_level = "Data";
my $second_level = "Dumper";
print Dumper *{${*{$::{$top_level."::"}}{HASH}}{$second_level."::"}}{H
+ASH}
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gives:

$VAR1 = {
          'Seen' => *Data::Dumper::Seen,
          'post' => *Data::Dumper::post,
          'Toaster' => *Data::Dumper::Toaster,
          'Varname' => *Data::Dumper::Varname,
          'pad' => *Data::Dumper::pad,
          'Values' => *Data::Dumper::Values,
          'Dumpf' => *Data::Dumper::Dumpf,
          'new' => *Data::Dumper::new,
          '_sortkeys' => *Data::Dumper::_sortkeys,
          'Pair' => *Data::Dumper::Pair,
          'DESTROY' => *Data::Dumper::DESTROY,
          'EXPORT_OK' => *Data::Dumper::EXPORT_OK,
          'Pad' => *Data::Dumper::Pad,
          'confess' => *Data::Dumper::confess,
          'Deepcopy' => *Data::Dumper::Deepcopy,
          'EXPORT_FAIL' => *Data::Dumper::EXPORT_FAIL,
          'Sortkeys' => *Data::Dumper::Sortkeys,
          'Freezer' => *Data::Dumper::Freezer,
          'ISA' => *Data::Dumper::ISA,
          'Terse' => *Data::Dumper::Terse,
          'Maxdepth' => *Data::Dumper::Maxdepth,
          'Dumpp' => *Data::Dumper::Dumpp,
          'Useperl' => *Data::Dumper::Useperl,
          'Dumpxs' => *Data::Dumper::Dumpxs,
          'bootstrap' => *Data::Dumper::bootstrap,
          '_dump' => *Data::Dumper::_dump,
          'qquote' => *Data::Dumper::qquote,
          'Useqq' => *Data::Dumper::Useqq,
          'BEGIN' => *Data::Dumper::BEGIN,
          'Purity' => *Data::Dumper::Purity,
          'Names' => *Data::Dumper::Names,
          'Deparse' => *Data::Dumper::Deparse,
          'EXPORT' => *Data::Dumper::EXPORT,
          'DumperX' => *Data::Dumper::DumperX,
          'croak' => *Data::Dumper::croak,
          'Dump' => *Data::Dumper::Dump,
          'import' => *Data::Dumper::import,
          'Indent' => *Data::Dumper::Indent,
          'Dumper' => *Data::Dumper::Dumper,
          'Quotekeys' => *Data::Dumper::Quotekeys,
          'Bless' => *Data::Dumper::Bless,
          'carp' => *Data::Dumper::carp,
          'VERSION' => *Data::Dumper::VERSION,
          'Reset' => *Data::Dumper::Reset,
          'Dumpperl' => *Data::Dumper::Dumpperl
        };
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and you can look up a particular symbol in that and use it. To call Data::Dumper::Dumper, for instance, you can say:

use Data::Dumper;
my $top_level = "Data";
my $second_level = "Dumper";
my $subname = "Dumper";
print &{*{${*{${*{$::{$top_level."::"}}{HASH}}{$second_level."::"}}{HA
+SH}}{$subname}}{CODE}}(\%Data::);
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calls Data::Dumper::Dumper via the symbol table and prints:

$VAR1 = {
          'Dumper::' => *{'Data::Dumper::'}
        };
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Actually, a lot of the above can be much simplified, because of an odd rule that says you can put a glob where a reference of a particular type is wanted, and it automatically produces the reference to that part of the glob:

use strict;
use Data::Dumper;
my $package_name = "Data";
print Dumper \%{$::{$package_name."::"}};

use strict;
use Data::Dumper;
my $top_level = "Data";
my $second_level = "Dumper";
print Dumper \%{${$::{$top_level."::"}}{$second_level."::"}};

use Data::Dumper;
my $top_level = "Data";
my $second_level = "Dumper";
my $subname = "Dumper";
print &{${${$::{$top_level."::"}}{$second_level."::"}}{$subname}}(\%Da
+ta::);
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