use strict;
use warnings;

print 'before package Foo ', exists($main::{'Foo::'}) 
    ? "exists" : "doesn't exist", "\n";
eval q{ package Foo };
print 'after package Foo ', exists($main::{'Foo::'}) 
    ? "exists" : "doesn't exist", "\n";

print 'before our Bar ', exists($main::{Bar}) 
    ? "exists" : "doesn't exist", "\n";
eval q{ our $Bar =99}; 
print 'after our Bar ', exists($main::{Bar}) 
    ? "exists" : "doesn't exist", "\n";

__END__
# produces
before package Foo doesn't exist
after package Foo exists
before our Bar doesn't exist
after our Bar exists