if (%h) {
      ...do something with a non-empty hash
   }

As far as I can tell, there is no method in the tied hash class that is invoked for this. Try the following module to test:

package TieTest;

use strict;
use Carp;

use vars qw(@ISA);

use Tie::Hash;
@ISA = qw( Tie::StdHash );

sub STORE    { notify('STORE');    shift->SUPER::STORE(@_);    }
sub FETCH    { notify('FETCH');    shift->SUPER::FETCH(@_);    }
sub FIRSTKEY { notify('FIRSTKEY'); shift->SUPER::FIRSTKEY(@_); }
sub NEXTKEY  { notify('NEXTKEY');  shift->SUPER::NEXTKEY(@_);  }
sub EXISTS   { notify('EXISTS');   shift->SUPER::EXISTS(@_);   }
sub DELETE   { notify('DELETE');   shift->SUPER::DELETE(@_);   }
sub CLEAR    { notify('CLEAR');    shift->SUPER::CLEAR(@_);    }

sub notify { print shift, " here.\n"; }

1;

   #!/usr/bin/perl
   use TieTest;
   tie %h, 'TieTest';
   grep($h{$_} = $_, 0 .. 9);
   print "Scalar test:\n";
   $c = %h;
   print "count is $c\n";