I'd be much more worried by identifiers of the form $fooN - use an array instead.

Assuming that you really intended $foo, $bar and $baz as the variable names I'd suggest the interpolation variants (two and three, without {} except where required) in most cases. Context may dictate that the join fits better with the surrounding code at times though. Because of double quote interpolation and Perl's general string handling power the concatenation operator is seldom used so the first variant is least likely to be used.