use Sys::Mmap;

open MMM, '+<', '/tmp/mmap' or die $!;

mmap $str, 0, PROT_WRITE|PROT_READ, MAP_SHARED, MMM;

print $str, "\n"; # Here I get "Hello" from file /tmp/mmap

# Here I should write "Goodbye" to mmap file. How do it?

print $str, "\n"; # Here I must get "Goodbye"

munmap($str);

exit;