> perl -we 'use strict;use P; P ((undef and 1) ? "yes": "");'  
no
> perl -we 'use strict;use P; P ((undef or "") ? "yes": "no");' 
no
> perl -we 'use strict;use P; P ((undef xor "") ? "yes": "no");'
no
> perl -we 'use strict;use P; P ((undef eq "") ? "yes": "no");' 
Use of uninitialized value in string eq at -e line 1.
yes
 perl -we 'use strict;use P; P ((undef and 1) ? "yes": "no");'     
no
> perl -we 'use strict;use P; P ((undef and 1) ? "yes": "no");'
no
> perl -we 'use strict;use P; P ((2 or undef ) ? "yes": "no");'      yes
> perl -we 'use strict;use P; P ((2 xor undef ) ? "yes": "no");'
yes
perl -we 'use strict;use P; P ((2 ne undef ) ? "yes": "no");'   
Use of uninitialized value in string ne at -e line 1.
yes
Ishtar:/tmp> perl -we 'use strict;use P; P ((undef xor undef ) ? "yes": "no");' 
no
 perl -we 'use strict;use P; P ((not undef xor not undef ) ? "yes": "no");'
no
etc....