There are a number of things in Perl, where Perl provides just a thin layer over the system libraries. What it does itself, and what it leaves to the system varies from version to version, from platform to platform, and from configuration to configuration (it matters for instance whether you are using PerlIO or stdio - PerlIO wasn't an option in 5.004, and wasn't the default in 5.6, but is the default in 5.8.x (but you still can use stdio).

Certain things behave, or can behave different on different OSses. For details, see "perlport.pod". So while many perl scripts will run fine (and identical) on any platform perl runs on, some perl programs won't. So putting it in i386 makes sense.