I don't understand.. I was noting that the /o flag is only useful for compiling a regex once, to avoid recompiling it later. However, this is only a factor if the regex has an interpolated variable. In other words, if Perl notices that the regex will be constant between calls, it will just compile it once anyway. The /o flag does nothing in this case. If Perl sees any interpolated variables, it has to assume that this variable may be different between regex tests, so it recompiles it every time, unless the /o option is given.