package Foo;

use vars qw($str);
$str = "foo str";

package main; 

my $pack = "Foo::";
my $var = "str";
print "$_: ",(defined ${$main::{$pack}}{$_} ? 'yup' : 'nope'),$/
    for qw(str bar);
__END__
str: yup
bar: nope