use Carp;

sub push_hash (\%) {
  my $href = shift;
  my $obj = tied %$href;

  croak "must be a Tie::Hash::Stack tied hash"
    unless $obj and $obj->isa('Tie::Hash::Stack');

  push @$obj, {};
}


sub pop_hash (\%) {
  my $href = shift;
  my $obj = tied %$href;

  croak "must be a Tie::Hash::Stack tied hash"
    unless $obj and $obj->isa('Tie::Hash::Stack');

  pop @$obj;
}


# and in your program...

tie my(%stash), 'Tie::Hash::Stack';

# ...
push_hash %stash;
# ...