@new= sort {
  my ($a1,$a2)= split /\s+/,$a;
  my ($b1,$b2)= split /\s+/,$b;
  return($a1<=>$b1 or $a2 cmp $b2);
} @old;
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should do it (untested). To understand this just do 'perldoc -f sort' to read about the sort function.

UPDATE: Added the much needed 'sort' key word after getting hints from almut, johngg and shmem