A valid question, since awk is a subset of perl (functionally, not necessarily syntactically); also, the authors of perl have usually known awk (and sed, and C, and a host of other Unix tools) very well, and still decided to move on.
There are some things that perl has built-in support for that almost no version of awk can do without great difficulty (if at all); if you need to do these things, there may be no choice to make. for instance, no reasonable person would try to write a web server in awk instead of using perl or even C, if the actual socket programming has to be written in traditional awk. However, gawk 3.1.0's /inet and ftwalk's built-in networking primitives may remove this particular limitation.
However, there are some things in awk's favor compared to perl:
Tom Christiansen wrote in Message-ID: <3766d75e@cs.colorado.edu>
> Awk is a venerable, powerful, elegant, and simple tool that everyone > should know. Perl is a superset and child of awk, but has much more > power that comes at expense of sacrificing some of that simplicity.