I'm confident that an OS could be written in Perl; but I'n not clear what is meant by a "Perl OS".

Writing an OS in pure Perl would be impractical because of the time constraints imposed by various kernel routines. Pure Perl would be too slow. You'd have to write those routines (or at least the vital bits) in C or assembly language (or whatever's sufficiently fast).

But - what would a Perl OS even be?