I'm no authority on tied objects, or on overload ... but i think you are confussing the two.

By overloading 'bool' in your package, you can define the behavior of evaluating an object of that package in boolean context -- but I don't think that's what is happening in your code.

By tieing your obejct as a hash, only the TIE* methods for hashes are ever called on your object. When you say: "if (%hash)" it doesn't matter that you have overloaded 'bool' for your package, all that matters is that you are evaluating a hash in a bolean context, which is 'true' if-and-only-if that hash has any elements.

Consider these modifications to your script (and teh output)...

#!/usr/bin/perl -wl
use strict;

my %hash ;
my $obj = new TiedHash();
tie(%hash , 'TiedHash') ;

print "checking Hash....";
print 'BOOLHASH' if ( %hash );

print "checking Object....";
print 'BOOLOBJ' if ( $obj );

package TiedHash ;

use overload
      'bool' => sub{ print "BOOL"; return 1 ; } ,
      '""' =>   sub{ print "STRING"; return 'tiedhash' ; } ,
      ;
sub TIEHASH { bless({}, __PACKAGE__ ) ;}
sub new { return TIEHASH; }
__END__

checking Hash....
checking Object....
BOOL
BOOLOBJ
[download]

...I'm not sure what methods on your tied hash are getting called when %hash is evaluated in boolean context, but i'm sure if you override all of the TIE* methods, you can figure it out.