It's easier to port a perl program then a shell script.

OTOH, there's less need to port a shell script than a Perl program. I've never had the need to port a shell script written in the Bourne shell to the C-shell, or from ksh to zsh. And I cannot recall ever to have written a shell program (and I have written many) that didn't work on a version of the shell five years old.

However, every now and then, I do need to adapt a Perl program because it needs to run on an older version, or because it no longer runs (or no longer runs warnings-free) on a newer version of Perl.

Furthermore, any Unix will have a shell that's Bourne shell compatible. Not every Unix will have Perl installed - specially not during OS-installation phase.