The qr// operator would be pretty lame if it didn't honor the modifiers that you compiled it with.

Yes, but that's a fair bit different than forcibly disabling the modifiers that you omitted...

What's surprising is not that qr/perl/i produces (?i:perl), but that it produces (?i-xsm:perl), with the other modifiers explicitly disabled.

That makes plenty of sense, but I'd never seen it explicitly documented before.

but isn't this sort of obvious?

Well, this is the whole point. It could not be ovbious for everyone. What is not clearly explained is that qr// will unset the modifiers that we haven't explicitly set.

I didn't mean to rewrite the Bible :), but just to warn against something that may not be immediately perceived. The problem only exists when you embed a pre-compiled regex within a larger expression.

 _  _ _  _  
(_|| | |(_|><
 _|   

Well, people should read documentation. If you see "qr" and don't understand what it does, the first thing you should do is read the docs -- don't just assume you know how it works.

From perldoc -f qr

       qr/STRING/

       qx/STRING/

       qw/STRING/
               Generalized quotes.  See the Regexp Quote-Like
               Operators entry in the perlop manpage.

From perldoc perlop

       qr/STRING/imosx
               This operators quotes--and compiles--its STRING as
               a regular expression.  STRING is interpolated the
               same way as PATTERN in `m/PATTERN/'.  If "'" is
               used as the delimiter, no interpolation is done.
               Returns a Perl value which may be used instead of
               the corresponding `/STRING/imosx' expression.