my $x = *Time::HiRes::gettimeofday;
print "original: ", Time::HiRes::gettimeofday(), "\n";
# ...
*Time::HiRes::gettimeofday = sub() { $x; };
                             #  ^^ avoids "Prototype mismatch"
print "restored: ", Time::HiRes::gettimeofday(), "\n";