sub my_header {
    # ... do your own stuff:
    print "Got parameters @_\n";
    # Call old header routine to output other stuff
    header();
};

my $pdf = PDF::API2::Simple->new( 
  header => \&my_header,
  footer =>\&footer,
  margin_left => 15,
  margin_top => 15,
  margin_right => 15,
  margin_bottom => 45
);