"The placement of a sub is important only when you call it without parentheses, ..."

I disagree with the inclusion of the word only in that statement. Here's a situation where placement is important when parentheses are used:

$ perl -wE '
    sub meaning () { 42 }
    say meaning();
'
42
[download]

$ perl -wE '
    say meaning();
    sub meaning () { 42 }
'
main::meaning() called too early to check prototype at -e line 2.
42
[download]

You can, of course, predeclare and leave the definition for later:

$ perl -wE '
    sub meaning ();
    say meaning();
    # ... later in code with other subroutine defintions
    sub meaning () { 42 }
'
42
[download]