$mw->Button(
        -text => "Submit",
        #-command => sub {exit} # no good, will exit whole program
         -command => sub{ $mw->destroy }   # good 
)->pack();

##</code><code>##

....  Tk code omitted
MainLoop;
# after running $mw->destroy, the following lines will get processed here, right after the MainLoop line

# if you have an external program to feed $text to, try one of these

system( "path_to_program  $text");
#or
exec ( "path_to_program  $text");

# if you want to run your program and get ouput

use IPC::Open3;
my $pid = open3(\*WRITE, \*READ, \*ERROR,"your_command");
if( ! $pid ){ die "$!\n";}

#send $text to your_command's STDIN
print WRITE "$text\n";

#get the answer from your_command
chomp(my $answer = <READ>);
print "response = $answer\n";

#get the error from your_command
chomp(my $error = <ERROR>);
print "error = $error\n";