No, it isn't random. Yes, you shouldn't depend on it.

In this case, I'm willing to bet that Perl has code that tries to seek, perhaps to more portably flush buffers. It is perfectly acceptable to try to seek and then see whehter or not it worked in order to tell if you happen to have a file handle to a stream that supports seeking.

$! actually starts out as zero in a new process (no, I don't think this is guarenteed anywhere) but quickly gets changed when something deep in the guts of some library does something that fails.

And indeterminate means it could be undef or "Illegal seek" or anything else.