The gotcha is that any attempts to use the builtins inside your module: return, split, delete, will result in calls to the subs in your package, not the builtins [...]

I can't find a test case for this behaviour - do you have an example?

There is a distinction between strong and weak keywords ¹ which shows a slight difference in behaviour. With a strong keyword, the keyword overrides the subroutine:

  zen% perl -wle 'my %hash; sub delete { die "not this one" } delete($
+hash{key})'
  zen%
[download]

A weak keyword gives a warning, but is still preferred to the subroutine:

  zen% perl -wle 'sub bless { die "not this one" } my $a=bless({});'
  Ambiguous call resolved as CORE::bless(), qualify as such or use & a
+t -e line 1.
  zen%
[download]

I'm sure I'm missing something that would justify your comment, but I can't offhand think what it is.

Hugo

¹ The distinction between weak and strong keywords is probably not well documented; in older perls I think the only way to check was to examine the C code in toke.c, but happily perl-5.8.7 now bundle a script perl_keyword.pl to generate the code, which has clear lists of weak (@neg) and strong (@pos) keywords at the start of the script.