The concatenation operator takes a scalar on both sides.

This is correct as far as it goes, but the truth is more specific...

The most peculiar thing I can think of in all this is the fact that concatenation operator doesn't accept a list while variable interpolation into a string does. I know that "concatenating" lists is what join is for. But to a relative greenie like me, it seems inconsistant.

You're missing a small bit of the contextual richness that makes Perl so cool. The binary concatenation operator evaluates its left and right arguments in string context, which happens to be a particular variety of scalar context. You can see this difference for example if you concatenate $!

$! = rand(7);  print("In numeric context it's ".
   ($!+0)." but in string context it's " . $! . $/);
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The join function is a list operator, like print. It evaluates each argument in string context, but it takes those arguments as a flattened list. You can write your own function that does this:

  sub listop {
     my @result;
     for (@_) {
        push @result, "".$_;
     }
     @result;
  }
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You can pass this thing an array or list (or several arrays or lists), and it will return a list containing all the elements, with each one stringified. This is basically what join and print do, except that rather than returning that list they do something interesting with it.

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
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