Anything that unambiguously uses a package automatically declares a package, and so does a package Foo statement.

So you could do

package Foo;
package main; # or whatever package is the 'current' one
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Which is probably the clearest way of doing it, or you can do something like:

$Foo::a=$Foo::a;  # using $Foo::a twice will silence the 'used only on
+ce' warning
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Note that fully specified package variables are exempt from 'strict "vars"' rules. Or you can declare a subroutine:

sub Foo::declare_package_Foo;
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The same technique would work for anything else that unambiguously uses the Foo package.