bench.pl: Correct optional = in pod synopsis
bench.pl: Load IO::File explicitly

to work with older perls that don’t load it automatically.

$ Porting/bench.pl perl5.8.8 perl5.8.7
Can't locate object method "blocking" via package "IO::File" at Porting/bench.pl line 737.

(In this case the system perl is 5.12.4.)