Maybe this is to avoid the "Looks like a function" problem described in perlfunc. If you always call your functions followed by +, you probably won't fall into that trap, even when editing without care (note that under warnings perl will warn you if you make such a mistake). That's all that an unary + does anyway, it has no effect on its operand, and just changes the way perl interpretes a statement.

Also note that this + is interpreted as unary because say accepts arguments, all functions that are declared to take none (empty prototype) will turn it into an addition:

use v5.14;
use warnings;
use strict;

sub my_say()
{
  say $_[0] || "No arguments!";
}

say (1+2)*3; # Same as (say(1+2))*3;
say+(1+2)*3; # Same as say((1+2)*3);
my_say+(1+2)*3; # Same as my_say()+(1+2)*3;
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say (...) interpreted as function at test.pl line 10.
Useless use of multiplication (*) in void context at test.pl line 10.
Useless use of addition (+) in void context at test.pl line 12.
3
9
No arguments!
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