I don't now how to get the pid of the tail process. Any suggestions?

$fh = FileHandle->new("tail -f -n 25 command_file |");

The only way to get the $pid is to use a piped-open as described in perldoc perlipc, or use something like IPC::Open3, but you sometimes get a shell pid, which then executes your program.

So use something like this:

my $fh = new FileHandle;
my $pid = open( $fh , "tail -f -n 25 command_file  | ") or die ;
#.....
# now you can check the output of ps to see if you get a shell $pid 
# or just your process $pid
# then to make sure you kill all shells and commands, if you see
# the $pid is the shell

use Proc::Killfam;
$mw->protocol('WM_DELETE_WINDOW' => sub {
                       killfam 9,$pid;
                       exit;
                      });
# or put the killfam in the callback of an Exit button, etc
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Before I get slapped on the wrist for using killfam 9, remember that 9 is an instant kill, you may want to use a softer signal like 15, to allow the program to gracefully close.