See Image and Text Watermarked Letters. For CGI purposes, just write to stdout as shown in the code below.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $image = Image::Magick->new;
my $wimage = Image::Magick->new;

my $rc = $image->Read('2uni2.jpg');  # base image
$rc = $wimage->Read(shift);  # image to be used as watermark

$rc = $image->Composite(gravity => "Center", 
     compose=>'Dissolve',
     image => $wimage,
      opacity => '10%',
      tile  => 1,  
      );

binmode STDOUT;
$image->Write('png:-');

# or to file for testing
#$image->Write("$0.png");

exit;
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