BTW, am I right in believing that the logic is up/back for outer sub-blocks ie greps, but fwd/down for sub-block contents?

Yes, essentially. Really perl reads everything from left-to-right, but it does "assignment" from right to left. With the usual formatting, it feels like the logical lines (seperated by semicolons) are being processed from top-to-bottom, but when you've got long chains of map/grep operations, it's traditional to put in line breaks, which makes it look like it's reading bottom-to-top, though really it's doing multiple assignments at once, going from right-to-left. Does that make sense?

A "Schwartzian" operation (sorting on a computed field), might seem like it's working bottom-to-top:

  @result = 
    map  { ... 3 ... }
    sort { ... 2 ... }
    map  { ... 1 ... };
[download]

But really it's doing assignment from right-to-left:

  @result = map  { ... 3 ... } sort { ... 2 ... } map  { ... 1 ... };
[download]

It does seem jarring to encounter a multi-line block in something like a grep, because it feels like the order of processing is jumping around: the whole program goes top-to-bottom, assignment in each logical line goes right-to-left, but then if you slip in a block with multiple lines, the top-to-bottom flow takes over again.