my %lookingFor; 
# keys => different name of one subset
# values => array of one subset
my $fh = new FileHandle "< largeLogFile.log";

while (<$fh>) {
  foreach my $subset (keys %lookingFor) {
    foreach my $item (@{$subset}) {
      if (<$fh> =~ m/$item/) {
        my $writeFh = new FileHandle ">> myout.log";
        print $writeFh <$fh>;
      }
   }
}