use Tk;
use Tk::DialogBox;
[download]

Those modules will do what you need. The documentation for Tk is pretty good but I suggest looking into Mastering Perl/Tk from O'Reilly. The nice thing about Tk is that a well writen Tk script will work the same on all platforms.

Update:
I think after rereading your question that you were not refering to the Windowing system but to within a web browser. Could you please clarify what you are trying to pop up.

Dominos does have their 3 medium pizzas for $5.00 each deal going on... ;)
Useless trivia: In the 2004 Las Vegas phone book there are approximately 28 pages of ads for massage, but almost 200 for lawyers.

#!/usr/bin/perl
use Tk;
use Tk::DialogBox;

my $file = shift || $0;

my $mw = MainWindow->new;

my $dialog = $mw->DialogBox(
    -buttons => [qw/Ok Cancel/],
    -title   => "Enter New Value"
);

my $dialogT = $dialog->add("Text");
$dialogT->pack(qw/-padx 10 -pady 10/);

my $button = $mw->Button(
    -text    => "Get new value...",
    -command => [ \&getNewValue, $file ]
)->pack(qw/-side left -padx 10/);

MainLoop;

sub getNewValue {

    my $filein = shift;
    open(FH,"<$filein") or warn "$!\n";
    my $text = do {local $/; <FH>};

    $dialogT->insert('end',$text);


## Determine whether or not the user hit "Ok" 
    my $button = $dialog->Show();
    if ( $button eq "Ok" ) {
        my $letters = $dialogT->get( '1.0', 'end' );
        print "$letters was submitted\n";

    }
}
__END__
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