BEGIN {
    sub main_package_vars {
        foreach(@_) {
            eval "sub $_ {
                my \$p = shift;
                \$p->{$_} = \$_[0] if \@_;
                \$p->{$_};
            }"
        }
    }
}
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Why do you put this in a BEGIN block? It serves no purpose, at least when Perl looks at it.

Also, why are you using string-eval, when the plain eval works just as well. Also see Class::Accessor.

    sub main_package_vars {
        foreach my $name (@_) {
            no strict 'refs'; *{ $name } = sub {
                my $p = shift;
                $p->{$name} = $_[0] if @_;
                $p->{$name};
            }"
        }
    }
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In this next snippet from MyPackage, I don't understand why you use string-eval, and also why you try to execute the same statement twice:

BEGIN {
    eval "*package_vars2 = \*main::main_package_vars";
    *package_vars3 = \*main::main_package_vars;
}
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The subroutine package_vars3 does not exist:

package_vars3( qw(one two three) );
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What problem are you trying to solve? Do you want to have a hash using which you get/set variables?

Maybe an object based on a plain hash using AUTOLOAD is already enough? If you want control over what keys get stored, again, see Class::Accessor.</c>