All the solutions proposed are excellent. The symbol table manipulation you proposed failed because you assigned the scalar $gettimeofday from Time::HiRes (which probably is not defined anyway). Instead of

*gettimeofday = \$Time::HiRes::gettimeofday;

you have to use "&" to get the subroutine reference:

*gettimeofday = \&Time::HiRes::gettimeofday;

Alternatively, you could assign the entire symbol table entry using "*":

*gettimeofday = \*Time::HiRes::gettimeofday;

Then not only would your module get Time::HiRes's "gettimeofday" subroutine, but also any other Perl things (array, hash, etc) with the name "gettimeofday".