Perl 4 and the book's first edition were released simultaneously, and Perl 4 was merely a version of the Perl 3 interpreter.
Depending on who you ask, this decision was either for convenience ("this book teaches Perl 4" is easier to say than "this book teaches Perl 3.numbers.here") or marketing ("look--Perl 4 came out!"), the latter sort of like Slackware's increment of version numbers from 4 to 7.
> the latter sort of like Slackware's increment of version numbers from 4 to 7
That was done because all of the other distros at the time had higher version numbers and people were looking at Slackware with a, "Well, it's only version 4. RedHat/Mandrake/etc are already on version 7!" attitude. I remember there being a written (web) justification for it.
Depending on who you ask, this decision was either for convenience ("this book teaches Perl 4" is easier to say than "this book teaches Perl 3.numbers.here") or marketing ("look--Perl 4 came out!"), the latter sort of like Slackware's increment of version numbers from 4 to 7.