Try using 'Plus' composition option, for example:

#!/usr/bin/perl

use Image::Magick;
use LWP::Simple;

LWP::Simple::mirror('http://s.a.cnn.net/si/features/2004_swimsuit/imag
+es/gallery/popup/may_03.jpg', './base.jpg');

$base = Image::Magick->new();
$base->Read('./base.jpg');
($width, $height) = $base->Get('width', 'height');

$watermark = Image::Magick->new();
$watermark->Set(size => "${width}x${height}");
$watermark->ReadImage('xc:white');
$watermark->Transparent( color => white );
$watermark->Annotate(pointsize => 36,
                     fill      => 'red',
                     text      => '1st Italian Perl Workshop\nPisa, 19
+ e 20 Luglio 2004',
                     gravity   => 'center');

$watermark->Composite(image   => $base,
                      compose => 'Plus',
                      gravity => 'Center');

$watermark->Write('./composition.jpg');
$watermark->Display;
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