thank you for that "form"-hint. that was what i needed. i finally assembled a script which does perfectly what i wanted to achieve:

i know that the string concatenation ($string) is a bit clumsy, but i wanted to add the comments. you cannot have comments in quotes, can you?

use strict;

use PDF::API2;
use PDF::Reuse;

# specify files
my $ifile = "c:/kopie/anlage-h-beg.pdf";
my $ofile = "c:/kopie/new.pdf";

#find out number of pages
my $ipdf  = PDF::API2->open($ifile);
my $nop   = $ipdf->pages;
$ipdf->end();

#set outfile for PDF::Reuse
prFile($ofile);

#get font
my $intName = prFont("Helvetica");

# assemble watermark text
my $string = 

# save graphic state
"q\n" .

# Begin Text
"BT\n" .

# set font and "size" 
"/$intName 1 Tf\n" .

# set a text matrix
# 1. width of text
# 2. rotation angle (counterclockwise)
# 3. inclination (clockwise)
# 4. height of text
# 5. position x (0=left)
# 6. position y (0=bottom)    
"180 60 -60 180 100 185 Tm\n" .        

#set rgb-color (values between 0 and 1)
#"1 0 0 rg" .

#set grayscale (white=1 black=0)
"0.9 g\n" .

# show text
"(Kopie) Tj\n" .

# End Text
"ET\n" .
"Q\n";


#loop over pages
for (my $i=1;$i<=$nop;$i++)
{
    #add watermark    
    prAdd ($string);

    #add original content    
    my $internalName = prForm 
    ( 
        {
            file     => $ifile,
            page     => $i,
            effect   => 'print',
            tolerant  => 'no'
        } 
    );
    
    #add pagebreak
    prPage;
}

#save and close
prEnd();
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