I'm not sure whether your callback ever gets passed the current page number. The best is to wrap the header subroutine with your own header:

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
);
[download]

Still can't pass the parameter. I'm trying this:

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

sub my_header
{
    my $pdf = shift;
    my $cat = shift;
    
    print $cat;
    
    header($pdf);
}

sub header {
  my $pdf = shift;
  # do my stuff
}
[download]

Erorr: Not a CODE reference at...

No. That can't work. That's also not what I showed. You will need to supply that parameter yourself, outside of the "parameters". A subroutine reference never takes parameters. Maybe something like the following:

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

sub my_header
{
    my $pdf = shift;
    my $cat = $CategoryName;
    
    print $cat;
    
    header($pdf);
}

sub header {
  my $pdf = shift;
  # do my stuff
}
[download]

... or, alternatively, if your $CategoryName changes several times before your (different) PDF objects output themselves, you can create the subroutines anonymously on the fly:

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

sub header {
  my $pdf = shift;
  # do my stuff
}
[download]