There is a brand new patch that speeds up repeated calls to unshift by a very large margin. This came out of the PerlMonks conversation at japhy looks at av.c (not av.h), and my original interest in this came out of a chatter conversation some time ago.

What I find particularly amusing is that this patch is based on making unshift bigger, slower, and wasteful of memory. Optimizations are certainly not always obvious! The way it works is that when you call unshift and it needs to extend your array, it adds to the request the size of the current array. The reason is that extending the array at the front involves moving the whole thing, and this is expensive so you want to do it as seldom as possible. In big-O notation with this change

push @foo, 1 for 1..$n;
takes O(n) instead of O(n*n). :-)

The patch has been accepted.

It is in the current development snapshot.