Not only will $0 be there, you can read and write to it as well:

package blanket;

use strict;

END
{
   open (IN, '<', "$0") || die "$0: $!";
 
   my $new = "$0.new";
   open (OUT, '>', "$new") || die "$new $!";

   while (<IN>)
   {
      print OUT map {' ' x (ord() - 1), "\n"} (split '')      
   }
   
   close IN;
   close OUT;
   
   rename $new,$0
}

1;

package unblanket;

use strict;

BEGIN
{
   open (IN, '<', "$0") || die "$0: $!";
 
   my $new = "$0.new";
   open (OUT, '>', "$new") || die "$new $!";

   while (<IN>)
   {
      print OUT chr(length($_))
   }   
   
   close IN;
   close OUT;
   
   do "$new";
   unlink "$new";
}

1;
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