Was test3 suppose to actually run in the terminal?

I do not think what you wrote will work this way (it didn't for me).

However, if we change test3.pl to produce something simple with a delay (sleep) like this:

#!/usr/bin/perl -w
# the new test3.pl
use strict;

$|=1; #NECESSARY

print "Write something: \n";

my $it=1;
until ($it == 5) {
        print "line $it...\n";
        sleep 1; $it++
}
[download]

Then your problem becomes comprehensible: run alone, this test3 produces five lines of text 1 second apart, but run from test.pl (the Tk) you wait five seconds and then see the five lines.

In addition to adding "$|=1;" to test3.pl, you need to add this Tk command to test.pl, in the loop after the text insert:

        $text_box -> insert ("end", $_);
        $main->update;
[download]

Now you can watch the five lines appear one at a time, the same way they would if they were logging a time-consuming process.