void rmsp(SV * sv) {
  char * end;
  int length;
  if( !SvPOK(sv) ) return;
  end = SvEND(sv);
  length = SvCUR(sv);
  end--; /* skip \0 */
  while( *end == ' ' && length >=0 ) { 
    end--;
    length--;
  }
  if( length >= 0 ) SvCUR_set(sv, length);
}  

##</code><code>##

Benchmark: timing 1000000 iterations of inline, regex...
    inline:  6 wallclock secs ( 4.19 usr +  0.01 sys =  4.20 CPU)
     regex:  9 wallclock secs ( 7.71 usr +  0.02 sys =  7.73 CPU)