You are probably way on your way with this, but I figure I would give you two examples in one. The first you will see is the Tk::FileSelect, which brings up a browser window for the user to select a file from (I prefer this solutions, simply because history has shown my not to trust the user). However, if what you are trying to do is a little more complex than you can simply implement this by using Tk::Entry, which is an easier solution than using a Textbox.
#!/usr/bin/perl -w
use Tk;
use Tk::FileSelect;
use strict;
my $mw = MainWindow->new;
#Solution #1 the next two lines!
my $FSref = $mw->FileSelect(-directory=>"C:/");
my $dir = $FSref->Show;
#solution #2 up until MainLoop
$mw->Label(-text => 'File Name: ')->pack;
my $filename = $mw->Entry(-width => 50, -textvariable => "$dir");
$filename->pack;
$mw->Button(
-text => 'Print',
-command => sub{do_print($filename)}
)->pack;
MainLoop;
#used by solution #2, to print the output to the terminal
sub do_print {
my ($file) = @_;
my $file_val = $file->get;
print "You selected $file_val\n";
}