my %exponent = (
  T  => 12,
  G  => 9 ,
  M  => 6 ,
  k  => 3 ,
  p  => -12,
  n  => -9,
  u  => -6,
  m  => -3,
);
s/\d(\w)\s/exists $exponent{$1} ? 'e' . $exponent{$1} : '' /ge;