open(FILE, "<", "numb.txt" ) || die "Unable to open numb.txt <$!>\n"; while ( <FILE> ) { chomp; $fileHash{$_} = $i++; } close(FILE); open(FILE, "<", "num.txt" ) || die "Unable to open num.txt<$!>\n"; while( <FILE> ) { chomp; if( exists $fileHash{$_} ) { } else { print "$_\n"; } } close(FILE); Kindly help me with this code so that I can append or write (output) the string that is selected onto the second file. Thanks.

You have opened the second file for read. You need to open for read / write. http://www.perlfect.com/articles/pe... You should name to two file handles differently to avoid confusion. To append to a file you have open for write, you can do something like open FILE, ">filename.txt" or die $!; print FILE "String" ; close FILE;

To append to an existing file you need to open it in append mode, which is '>>'. If you use '>', that will overwrite the file, which means you'll loos any data that was already in the file. Here's some comments on the code. You're using the 3 arg form of open, which is good, but you should also be using a lexical var for the filehandle instead of the bareword and my personal preference is to use the lower precedence or instead of ||. open my $FILE, '>>', 'num.txt' or die "Unable to open num.txt <$!>\n"; Having an if block that does nothing is pointless. Just reverse the logic and print. while( <$FILE> ) { chomp; print "$_\n" if not exists $fileHash{$_}; }