You have several options:

• Run the file through an output filter (such as you might find on Unix or Cygwin) that will change it to landscape mode, then copy it to the printer.
• Prepend a printer control language command to the file to put the printer in landscape mode. The appropriate command will depend on the make and model of the printer.
• Create a printer queue that defaults to printing in landscape mode, then print to that instead.

None of these solutions are Perl specific.