since a regular expression without variable interpolation doesn't have the potential to change between uses, Perl only ever compiles it once. however, when a regular expression contains a variable to be interpolated, the value of the variable can potentially be different every time, so Perl compiles the regular expression with the current value of the variable every time it is used. the 'o' modifier tells Perl that the values of interpolated variables should be treated as constant, so that Perl will only compile the regular expression 'once' no matter how many times you use it.

i'm not sure whether or not using 'o' on a regexp without any variable interpolation adversely affects performance, but it is simply unnecessary.