use threads;
use threads::shared;

my $thr; # thread
my $Rval:shared;# return value inside thread
my $RTval; # return value intended to be "out in Tk code"

# the following 3 lines of code would be invoked by a button press
$thr=threads->new(\&sub_name);
$TRval = $thr->join;
print "$TRval"; # $TRval is in MainWindow Tk code

sub sub_name
{
   $Rval = `a_command_line_function`; # final code will have serial port activity
}