s{(^)?\s+(\z)?}{ defined $1 || defined $2 ? '' : ' ' }gex

##</code><code>##

 $ perl5.8.3 -del
  DB<1> x "hi" =~ /i\z/
0  1
  DB<2> x "hi" =~ /(i)(\z)?/
0  'i'
1  undef

 $ perl5.10 -del
  DB<1> x "hi" =~ /(i)(\z)?/
0  'i'
1  ''