The units used in PDF::Create are the same as Adobe uses for PDF and Postscript a unit called 'point'. This unit represents the smallest addressable unit in a 72DPI device. As a result a A4 portrait page is 595 points wide and 842 point high. These numbers are used to define the A4 page size in the get_page_size("A4") subroutine.
These numbers should be your limits when drawing on a A4 page.