It should be $_ like the others explained. Let's look at why it doesn't work with $_. (I don't see how it would work with $- any better.) You're missing parens in your first match. m// returns 1 or undef/() if you have no captures.

{
  my @new;
  $_ = 'bla';
  push @new, $_ =~ m/^bla/;
  print(@new, "\n");  # prints 1
}

{
  my @new;
  $_ = 'bla';
  push @new, $_ =~ m/^(bla)/;
  print(@new, "\n");  # prints bla
}
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$_ =~ is redundant and can simply be ommited. Also, s/...(.*)/...$1/ is the same as s/.../.../. The final product is:

while (<IN>) {
   push @new, m/^(\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+)/;
   print "\n$#num\n";
}

while (<REPLACE>) {
   s/^\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+/$new[$i++]/;
   print OUT $_;
}
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