I learned that you should not combine pdf files with different pdf versions. They can be converted by reinterpreting them in ghostscript, as below.

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4
            -dNOPAUSE -dQUIET -dBATCH
            -sOutputFile=clean.pdf original.pdf
# In windows, replace 'gs' with 'gswin32c.exe'
[download]

that made the trick for me. It's trivial to call ghostscrip with 'exec' or 'system' from perl.

It wasn't a perl problem and I should not have posted it in perlmonks, probably, but I wanted to share the solution anyway.

Cheers

Giovanni