A technique which nobody has mentioned above is simply comparing against a control group. Time a few thousand iterations loading the module,

% for e in `seq 1000` ; do perl -Mmymodule -e1 ; done
[download]

Then time a few thousand iterations of not loading the module.

% for e in `seq 1000` ; do perl -e1 ; done
[download]

Subtract the control group timing from the target group timing. What difference do you find?