...hopefully this does what you need

use strict;
use warnings;

open(OUT, '>', "out.txt");

open(FH, "test.res") or die "$!";
while(<FH>){
  chomp;
  if($_ =~ m/--gnu.+-D/){
    my @words = split(/\s+/, $_);
    foreach my $word(@words){
      if($word =~ m/^-D/){
        print OUT "$word\t";
      }
    }
    last;
  }
}
close(FH);
close(OUT);
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