my $final_header;
      open (FH, "<", $input_file) or die "Can't open $input_file: $!";
      while (<FH>) {
          chomp;
         if ($. == 1) {
              $final_header = sprintf("%s\t%s\t%s\n", join("\t", @left), $_, join("\t",@right));
              last;
          }
      }
      close (FH) or die "Can't close $input_file: $!";
...
...    
...

...
      close (FH) or die "Can't close $input_file: $!";
    
       # Once the final header is set, print it to the output file
      open (FH, ">", $output_file) or die "Can't open $output_file: $!";
      print FH $final_header;
      close (FH) or die "Can't close $output_file: $!";
      
            
      # capture data
      my $row_data;
      open (RD, "<", $input_file) or die "Could not open $input_file: $!";

while( my $line = <RD>)  {   
    print $line;    
    last if $. == 2;
}