Declares the BLOCK, or the rest of the compilation unit, as being in the given namespace. The scope of the package declaration is either the supplied code BLOCK or, in the absence of a BLOCK, from the declaration itself through the end of the enclosing block, file, or eval (the same as the my operator). All unqualified dynamic identifiers in this scope will be in the given namespace, except where overridden by another package declaration.
A package statement affects dynamic variables only, including those you've used local on, but not lexical variables, which are created with my (or our (or state)). Typically it would be the first declaration in a file included by require or use. You can switch into a package in more than one place, since this only determines which default symbol table the compiler uses for the rest of that block. You can refer to identifiers in other packages than the current one by prefixing the identifier with the package name and a double colon, as in $SomePack::var or ThatPack::INPUT_HANDLE. If package name is omitted, the main package as assumed. That is, $::sail is equivalent to $main::sail (as well as to $main'sail, still seen in ancient code, mostly from Perl 4).
If VERSION is provided, package sets the $VERSION variable in the given namespace to a version object with the VERSION provided. VERSION must be a "strict" style version number as defined by the version module: a positive decimal number (integer or decimal-fraction) without exponentiation or else a dotted-decimal v-string with a leading 'v' character and at least three components. You should set $VERSION only once per package.
See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues.