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use strict;
use warnings;
...

   inner();
}

>perl -c test.pl
Variable "$foo" will not stay shared at test.pl line 8.
test.pl syntax OK

use strict;
use warnings;
...

   $inner->();
}

>perl -c test.pl
test.pl syntax OK

use strict;
use warnings;
...
      $inner->()
   }
}

Global symbol "$inner" requires explicit package name at test.pl line 
+6.
test.pl had compilation errors.

use strict;
use warnings;
...
      $inner->()
   }
}

>perl -c test.pl
test.pl syntax OK

# ReleaseTracker.pm

...
sub o { ReleaseTracker->new(); }

1;

use strict;
use warnings;
...
outer( o() );

END { print("On program exit:\n"); }

>perl test.pl
On program exit:
Destroyed           <--- BAD!

use strict;
use warnings;
...
outer( o() );

END { print("On program exit:\n"); }

>perl test.pl
Destroyed
On program exit:
                    <-- good

sub outer {
   ...
...
   };
   helper(@_);
}

sub outer {
   ...
...
      }
   } )->(@_);
}

sub outer {
   ...
...
   weaken($helper);
   $helper->(@_);
}