my $Line_QC = "/full/directory/path";


    opendir(DIR, "$Line_QC") or die $!;
          my @files_array 
             = grep { /^35_/ && -f "$Nav_Line_QC/098/$_"}
readdir(DIR);

   my $diag_file = quotemeta"@files_array[0]";



print "@diag_files_array[0]\n";
     # prints: 35_Diag_098_a098@421424012017.txt

print "$diag_file\n";            
     # prints: 35_Diag_098_a098\@421424012017\.txt


open ( DIAG, "$diag_file" ) ||die ("Failed: $!");
    while (my $line = <DIAG>) {
      if ($line =~ /Line/) {
             print "$line\n";
           };
    };              # Close while loop
close ( DIAG );