I don't see the point, but breaking it down:

• Read from file: <>
• Read in array context; i.e., get all lines from file: ()=<>
• Assign those read lines to $=. That's the page length for formatted output: $==()

Basically it's saying, make the format page length the size of the number of lines in file. But just printing that doesn't really do a whole lot unless I'm missing something ...