You can open a file for output and print to that outfile.

  open (MYOUTFILE, ">outfile.txt") or die "Can't create outfile: $!";

    # Other code

    print MYOUTFILE "$A,$B,n/a\n" if $i==0;
    print STDOUT "$A,$B,n/a\n" if $i==0;

    # Other code

  close MYOUTFILE;
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or

   if ($i==0){
      print MYOUTFILE "$A,$B,n/a\n";
      print STDOUT "$A,$B,n/a\n";
    }
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There are variations on this and you can include screen output, append to files, and more. Your best bet is to read up on filehandles and open and print.

Resources

• Open
• Print
• Perl FAQ 5 on I/O File Handles, Formats, etc.

Update: Closed off some tags that messed up the code and picked up some dropped end tags.