Hello jmeek, and welcome to the Monastery!

In Perl, the only truly global variables are those with predefined “punctuation” names: $/, $|, etc. (ok, also $a and $b). Other “global” variables (i.e., those with user-supplied names) are actually package-globals, meaning they are global within the package in which they are defined. In modern Perl, these variables are typically accessed in other packages via suitable our declarations, but the older mechanism of use vars is still available. Compare the following one-liners:

18:05 >perl -Mstrict -wE "package Foo; sub foo { say $x; } package mai
+n; $x = 42; Foo::foo;"
Global symbol "$x" requires explicit package name (did you forget to d
+eclare "my $x"?) at -e line 1.
Global symbol "$x" requires explicit package name (did you forget to d
+eclare "my $x"?) at -e line 1.
Bareword "Foo::foo" not allowed while "strict subs" in use at -e line 
+1.
Execution of -e aborted due to compilation errors.

18:06 >perl -Mstrict -wE "use vars qw( $x ); package Foo; sub foo { sa
+y $x; } package main; $x = 42; Foo::foo;"
Global symbol "$x" requires explicit package name (did you forget to d
+eclare "my $x"?) at -e line 1.
Bareword "Foo::foo" not allowed while "strict subs" in use at -e line 
+1.
Execution of -e aborted due to compilation errors.

18:06 >perl -Mstrict -wE "use vars qw( $x ); package Foo; sub foo { sa
+y $::x; } package main; $x = 42; Foo::foo;"
42

18:06 >perl -Mstrict -wE "package Foo; sub foo { say $::x; } package m
+ain; $x = 42; Foo::foo;"
Variable "$x" is not imported at -e line 1.
Global symbol "$x" requires explicit package name (did you forget to d
+eclare "my $x"?) at -e line 1.
Execution of -e aborted due to compilation errors.

18:06 >
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In all cases, $x is actually $main::x; a use vars declaration doesn’t change that. In the third case, supplying the package name removes the error and allows package Foo to access the variable although it’s from a different package. (The final case is given to illustrate the role use vars is playing in preventing errors from use strict.)

Hope that helps,

A very fine explanation of a difficult (to me, anyway) Perl concept.   Thanks for sharing.   ++

Nit: It actually refers the var in the root namespace, but main is aliased to the root namespace, so the following all refer to the same var:

$::var
$main::var
$main::main::var
$main::main::main::var
etc
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So @::foo is just shorthand for @main::foo, and the need for this syntax has been mostly rendered obsolete since it became possible to use our @foo; instead of use vars ( @foo );. (Right?) Many thanks.

No. Assuming the purpose is to "defeat" use strict;, it was made obsolete by use vars qw( @foo );.

perl -e'
   use strict;
   @foo = 123;
   print "ok\n";
'
Global symbol "@foo" requires explicit package name at -e line 3.
Execution of -e aborted due to compilation errors.

$ perl -e'
   use strict;
   use vars qw( @foo );
   @foo = 123;
   print "ok\n";
'
ok
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But since strict was introduced in Perl 5.000 and vars was introduced in Perl 5.002, it's safe to say that was not the book's reason for using @::foo.

In fact, it's not obsolete. You still need to use fully qualified names to refer to variables in namespaces other than the current one.

use Data::Dumper qw( Dumper );
local $Data::Dumper::Useqq = 1;
local $Data::Dumper::Sortkeys = 1;
print(Dumper($data));
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$Foo::Bar::baz refers to the package variable $baz in the namespace Foo::Bar.

Similarly, $::baz refers to the package variable $baz in the root namespace.

The default namespace, main, is an alias for the root namespace. So unless you have a package declaration changing the current namespace from the default, $::baz refers to package variable $baz in the current namespace.

Unless he used a package statement, I'm not sure why he used %::man instead of %man. My only guess is that it was to avoid the possibility of referencing a lexical (my) variable named %man by accident.