See the current Perl documentation for package.
Here is our local, out-dated (pre-5.6) version:
package - declare a separate global namespace
Declares the compilation unit as being in the given namespace. The scope of
the package declaration is from the declaration itself through the end of
the enclosing block (the same scope as the local() operator). All further unqualified dynamic identifiers will be in this namespace.
A package statement affects only dynamic variables--including those you've used
local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file to be included by
or use operator. You can switch into a package in more than one place; it merely
influences which symbol table is used by the compiler for the rest of that
block. You can refer to variables and filehandles in other packages by
prefixing the identifier with the package name and a double colon:
$Package::Variable. If the package name is null, the
package as assumed. That is,
$::sail is equivalent to
NAMESPACE is omitted, then there is no current
package, and all identifiers must be fully qualified or lexicals. This is
use strict, since it also extends to function names.