That's silly. Someone who doesn't know what join does or the arguments it takes--or who doesn't know how to look it up in a reference or in the Perl documentation--has no business maintaining a serious Perl program. While there are definitely Perl operators with poor interfaces (caller is one of my least favorite), avoiding the use of join and slices because people who don't know Perl might not understand it is silly.

(I realize you said "casual reader", which implies someone not reading carefully, but a similar argument applies there.)

The three strings to be concatenated are given in a different order than from left to right, which might be counter-intuitive for a casual reader.

Fat arrow to the rescue?

join $separator => @things{'this', 'that', 'the other'};
[download]

However, where the task at hand is better described as prepending and appending to a string, rather than sewing together a list of arbitrary length, I concur with some others in this topic that concatenation is clearer.

As a casual reader, albeit one that has used, and is used, to the join function, I had to go over the OP twice to understand what is said in the first option (join '',a,b,c). I can't really relate to using a function in a way that is counter intuitive to someone who is familiar with the way it should be used, but I'd hazard that for anyone who never used it, or Perl, neither will make sense; and if that person is to perldoc it, the first example might only serve to confuse more.