G'day Bod,

[Assumption of typos: first sentence (after quote) s/what are they included/why are they included/; last sentence s/situation has occurred yet/situation has not occurred yet/.]

"Well...that begs the question...whatwhy are they included in the language?"

The only time I use prototypes is when I want a subroutine to be inlined (and then only in rare cases). You can read about inlining in "perlsub: Constant Functions". Reproducing the test examples (a fair way down in that section):

$ perl -MO=Deparse -e 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
sub ONE {
    1;
}
if (ONE) {
    print ONE() if ONE;
}
-e syntax OK

$ perl -MO=Deparse -e 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
sub ONE () {
    1;
}
do {
    print 1
};
-e syntax OK
[download]

When subroutine signatures are enabled (see ++chromatic's earlier discussion; and also "perlsub: Signatures") there's a conflict with this syntax:

$ perl -MO=Deparse -e 'use v5.36; sub ONE () { 1 } if (ONE) { print ON
+E if ONE }'
sub BEGIN {
    require v5.36;
    ()
}
use warnings;
use strict;
no feature ':all';
use feature ':5.36';
sub ONE () {
    1;
}
if (ONE) {
    print ONE() if ONE;
}
-e syntax OK
[download]

You can resolve this by using the :prototype attribute (see "perlsub: Prototypes"):

$ perl -MO=Deparse -e 'use v5.36; sub ONE :prototype() { 1 } if (ONE) 
+{ print ONE if ONE }'
sub ONE () {
    1;
}
sub BEGIN {
    require v5.36;
    ()
}
use warnings;
use strict;
no feature ':all';
use feature ':5.36';
do {
    print 1
};
-e syntax OK
[download]

But don't also try to use a signature as that will break it again:

$ perl -MO=Deparse -e 'use v5.36; sub ONE :prototype() () { 1 } if (ON
+E) { print ONE if ONE }'
sub BEGIN {
    require v5.36;
    ()
}
use warnings;
use strict;
no feature ':all';
use feature ':5.36';
sub ONE : prototype() () {
    1;
}
if (ONE) {
    print ONE if ONE;
}
-e syntax OK
[download]

Now, I did say at the outset "only in rare cases". That's because, for the most part, the constant pragma does what I want regardless of whether signatures are enabled or not.

$ perl -MO=Deparse -e 'use constant ONE => 1; if (ONE) { print ONE if 
+ONE }'
use constant ('ONE', 1);
do {
    print 1
};
-e syntax OK

$ perl -MO=Deparse -e 'use v5.36; use constant ONE => 1; if (ONE) { pr
+int ONE if ONE }'
sub BEGIN {
    require v5.36;
    ()
}
use warnings;
use strict;
no feature ':all';
use feature ':5.36';
use constant ('ONE', 1);
do {
    print 1
};
-e syntax OK
[download]

In fact, needing an empty prototype to inline a subroutine is so rare, I can't immediately think of an example. Perhaps I'll come up with one later. :-)