Can you reduce the program to a very short program that still exhibits the problem? I'm thinking of something like:

#!perl
use strict;
use warnings;
use PDF::API2;

print "Using PDF::API2 version $PDF::API2::VERSION\n";

my $pdf = PDF::API2->new;
my $bg_file = 'watermark.pdf';
my $bg = $pdf->importPageIntoForm( $bg_file, 1 );

# ...

$page->gfx->formimage( $bg, 0, 0, 1.0 )

# write to another file
[download]

That would help me understand your steps better and maybe point out where the problem(s) might lie.