cperl-problems-old-emaxen is a variable defined in `cperl-mode.el'.
Its value is

Description of problems in CPerl mode specific for older Emacs versions.

Emacs had a _very_ restricted syntax parsing engine until version
20.1. Most problems below are corrected starting from this version of
Emacs, and all of them should be fixed in version 20.3. (Or apply
patches to Emacs 19.33/34 - see tips.) XEmacs was very backward in
this respect (until 2003).

Note that even with newer Emacsen in some very rare cases the details
of interaction of `font-lock' and syntaxification may be not cleaned
up yet. You may get slightly different colors basing on the order of
fontification and syntaxification. Say, the initial faces is correct,
but editing the buffer breaks this.

Even with older Emacsen CPerl mode tries to corrects some Emacs
misunderstandings, however, for efficiency reasons the degree of
correction is different for different operations. The partially
corrected problems are: POD sections, here-documents, regexps. The
operations are: highlighting, indentation, electric keywords, electric

This may be confusing, since the regexp s#//#/#; may be highlighted
as a comment, but it will be recognized as a regexp by the indentation
code. Or the opposite case, when a POD section is highlighted, but
may break the indentation of the following code (though indentation
should work if the balance of delimiters is not broken by POD).

The main trick (to make $ a "backslash") makes constructions like
${aaa} look like unbalanced braces. The only trick I can think of is
to insert it as $ {aaa} (valid in perl5, not in perl4).

Similar problems arise in regexps, when /(\s|$)/ should be rewritten
as /($|\s)/. Note that such a transposition is not always possible.

The solution is to upgrade your Emacs or patch an older one. Note
that Emacs 20.2 has some bugs related to `syntax-table' text
properties. Patches are available on the main CPerl download site,
and on CPAN.

If these bugs cannot be fixed on your machine (say, you have an inferior
environment and cannot recompile), you may still disable all the fancy stuff
via `cperl-use-syntax-table-text-property'.