Maybe if one doesn't know Perl well enough to simply use concatenation to join two items with a separator one is bound to get confuzzled about the order things come out of a join . . . :)

Update: Just to clarify, that's a big smiley there. I don't think the join form is unqualified blasphemy dooming the author to an eternity of maintaining phpbb mods, but it's not the first choice I'd use unless it was relatively certain that the number of things needing to be concatenated was either arbitrary or varying (e.g. you need to conditionally intersperse $config{'infix'} every fourth Thursday in months not containing an "r"). qq{$config{'prefix'}$sep$config{'suffix'}} reads just fine to me, as does the OP's concatenation version.