>perl -wMstrict -le
"sub Sr { return }
 sub Se { return () }
 sub Sw { return wantarray ? () : undef }
 print q{'}, Sr, q{'   '}, join(q{'   '}, 0+Sr, ''.Sr), q{'};
 print q{'}, Se, q{'   '}, join(q{'   '}, 0+Se, ''.Se), q{'};
 print q{'}, Sw, q{'   '}, join(q{'   '}, 0+Sw, ''.Sw), q{'};
"
Use of uninitialized value in addition (+) at -e line 1.
Use of uninitialized value in concatenation (.) or string ...
''   '0'   ''
Use of uninitialized value in addition (+) at -e line 1.
Use of uninitialized value in concatenation (.) or string ...
''   '0'   ''
Use of uninitialized value in addition (+) at -e line 1.
Use of uninitialized value in concatenation (.) or string ...
''   '0'   ''