Yes, that is one difference, but I find code that makes use of that difference to be a bit of a problem. I'd probably write HI_POST_PI()+5 if I noticed.

The difference that I'm talking about is that subroutines with a () prototype that also do nothing but return a value become compile-time constants. Perl can replace instances of calls to these with the constant returned at compile time, allowing lots of interesting tricks:

> Perl -MO=Deparse -w
use strict;
sub DEBUG() { 0 }
sub SymRef() { "My::Package::hash" }
sub PI() { 3.141592 }
if(  DEBUG  ) {
    warn "Okay, we are debugging...\n";
    do_lots_of_debugging_stuff();
}
$My::Package::hash{key}= "value";
print SymRef->{key},$/;
print PI()+5,$/;
__END__
sub DEBUG () {
    0;
}
sub SymRef () {
    'My::Package::hash';
}
sub PI () {
    3.141592;
}
'???';
$My::Package::hash{'key'} = 'value';
print $My::Package::hash{'key'}, $/;
print 8.141592, $/;
- syntax OK
[download]

sub DEBUG {
    0;
}
sub SymRef {
    'My::Package::hash';
}
sub PI {
    3.141592;
}
if (DEBUG ) {
    warn "Okay, we are debugging...\n";
    do_lots_of_debugging_stuff ;
}
$My::Package::hash{'key'} = 'value';
print SymRef()->{'key'}, $/;
print PI() + 5, $/;
- syntax OK
[download]

Can't use string ("My::Package::hash") as a HASH ref
while "strict refs" in use
[download]