this_thing.pl
#!/usr/bin/env perl 
use strict;
use warnings;
use utf8;
use Tk;

my $mw = MainWindow->new;
my $button = $mw->Button(
    -text => 'start other thing',
    -command => \&start,
)->pack;
my $text = $mw->Text()->pack;
MainLoop;

sub start {
    my $script_path = 'this_other_thing.pl';
    if (not my $read = do $script_path) {
        warn "couldn't parse $script_path: $@" if $@;
        warn "couldn't do $script_path: $!" unless defined $read;
        warn "couldn't run $script_path" unless $read;
    }
}

this_other_thing.pl
#!/usr/bin/env perl 
use strict;
use warnings;
use utf8;

print "Hello World!\n";