As I commented in To push and pop or not to push and pop?, there is almost no difference in the performance of shift, unshift, push and pop. This is not a place to look for efficiency gains - writers of the Perl code have already done it :).

A more efficient approach is to profile. Take a look at some of the answers in Using Devel::Profile output to know what to do next to get some ideas (found using Super Search for nodes whose titles contain "profile" in SoPW).