This almost worked. Unfortunately it mixed it up with an other element that has expand option, and this other element was sandwiched between the two buttons ( which were correctly placed left and right ). I had to introduce a dummy frame:

# dummy frame to contain buttons

  $self->{delete_frame}=$mw->Frame()
    ->pack(-expand => '1', -fill => 'both' );

# buttons
    
  $self->{delete_button}=$self->{delete_frame}->Button(-text=>'Delete'
+,-command=>[ \&delete , $self ])
    ->pack(-side=>'left', -expand => '1', -fill => 'both' );

  $self->{undelete_button}=$self->{delete_frame}->Button(-text=>'Undel
+ete',-command=>[ \&undelete , $self ])
    ->pack(-side=>'right', -expand => '1', -fill => 'both' );
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