s/(.*)(right)?/\1 <> \2/

echo "left"|perl -ne 'print if s/(.*)(right)?/\1 <> \2/'                left <>
echo "right"|perl -ne 'print if s/(.*)(right)?/\1 <> \2/'               right <>
echo "leftright"|perl -ne 'print if s/(.*)(right)?/\1 <> \2/'           leftright <>