Here is a trick I used using Tk. The Tk's Text clipboard runs pretty good, and you can use the copy/paste functions without actually showing the text box. You can run it, then use the right mouse button click to paste it here in the text box. It is a mediocre solution, but copying and pasting between applications can be tricky, because the application will usually clear the clipboard when they exit. Also, there are different clipboards. There is the mouse paste, the Paste in Menu's, etc. In other words, there is no simple answer.
Your best bet is to use something that has a clipboard, like Notepad, or Tk::Text, load your file into it, then copy it. I do something like that in
Gtk2 Visual Grep, which is what I use to copy code from files, into the Paste buffer for pasting here on Perlmonks. I use "vgrep n somefile", click on the name to load it, the use the rightclick menu to select all and copy.
Tk copy to mouse clipboard and
#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::Clipboard;
my $mw = tkinit;
$mw->withdraw; #use Tk without showing a window
my $content = 'foobar'.time;
print "$content\n";
$mw->clipboardClear;
$mw->clipboardAppend($content);
MainLoop;