I like the way you explained breaking out standard functions, from core functions. It's something I've always done, but have found myself hard pressed to enunciate as to why I do it. It has always stumbled out of my mouth as, "Cuz I don't want to mess the other files up".

I do have a small point of contention with a portion of your conclusion though. You said,

...changing the class interface (which is only slightly better than having no interface at all.)

Speaking from my position as a struggling Perl person that's learning both the base perl code and the many abstractions¹ of perl as 00. I can suck the power of a 2 processor SPARC 450 because of the horrendous programing in my applications core modules. For this reason I contend that,
Changing the class interface, is worse than no interface at all.

coreolyn Duct tape devotee.
-- That's OO perl, NOT uh-oh perl !-)

¹ From my slighlty crazed point of view, OO Perl takes Perl from being 'duct_tape' to 'silly putty on acid'.

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