All,
I am likely getting some of this wrong and I am sure some of the more knowledgeable monks will correct the inaccuracies. The question remains regardless.

The /o regex modifier is an old optimization before qr// existed. If you had a regex that needed variable interpolation before compiling but knew the variable would never change, you could use the /o modifier to only compile the regex once (/$regex/o). In fact, if you broke your promise and modified the variable, the regex still would not be recompiled leading to some potentially buggy code. See perlre for more information.

The qr// operator greatly improved things. See perlop for more details. In my opinion, this pretty much rended the need for /o moot. I am not the only one, diotalevi has /o is dead, long live qr//! to say on the matter as well as this.

Ok, I get it - backwards compatability and TIMTOWTDI. There is a lingering question. It is pointed out here that in newer versions, perl automatically detects when a variable is changed and only recompiles a regex when necessary. This is apparently not documented. I checked perldeltas from 5.6 to 5.9.4 though I may have missed it.

So why is this behavior not documented? Is there any reason not to deprecate /o? I am happy to perlbug a doc patch but I am certainly not the most qualified to do so.

Cheers - L~R

In reply to Questions concerning /o regex modifier by Limbic~Region

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