Hi, I think I found the answer for you. Change 'Dissolve' to 'Atop'. The following script works here like the shell version.

P.S. Don't feel like a dunce, I seem to remember spending a few hours trying every compose type when I discovered this a few years back. :-) I changed a few filenames and lowered the opacity for clarity.

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

#From the command line, this is the result I'd like to achieve:
# composite -dissolve 87.8 foreground.jpg -size 500x500 xc:white outpu
+t.jpg; 
# composite mask.png output.jpg output.jpg 

my $mask = Image::Magick->new(magick=>'png');
my $background = Image::Magick->new(magick=>'png');
my $foreground = Image::Magick->new(magick=>'png');

$mask->Read("png:zzzmask.png");

$background->Set(size=>'500x500');
$background->Read("xc:white");
$background->Set(magick=>'png'); 
            
$foreground->Read("zzzforeground.jpg");
$foreground->Set(magick=>'png'); 
            
$background->Composite(compose=>'Atop', image=>$foreground, opacity =>
+ '50.8%' ); 
$background->Composite(image=>$mask);
$background->Write("zzzpoutput.png");

exit; 

# The mask can be any png with transparency (any kind of watermark), 
# and foreground any 500x500 image.
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