You can use open using the pipe | at the end of the command to read from a command output and then parse each line.

Like in:

open my $cmd, "/bin/cmd|" or die "something useful here like $^E";

while (<$cmd>){
   chomp;
   print $append_file "$1\n" if $_ =~ /_id:(\S)+$/; #please re-check t
+he regex..
}
[download]

L*

PS in the same way you can write to a program STDIN putting the pipe in front of the program like open my $sendmail, "|/usr/sbin/sendmail etc etc";

The above example is the basic way to do what do you ask for but many modules on cpan can achieve the same in different, perhaps more robust, way.

Also note that you can open your append file once and outside of the loop: you reopen it many times and you profit of the automatic close when $file goes out of the scope: do not take this habit! use the explicit close as soon as you do not need the filehandle anymore.

More: if you just want to append to a single file during all the program you can use select $filehandle to have all print calls redirected to such file.

L*