tye provided the answer in CB
<tye> as to /why/ that distinction matters: it is a parser restriction (and parsing sort arguments is rather buggy anyway)
<tye> perl -MO=Deparse -e "sort foo(), <>" says "sort foo (), <ARGV>;", which leads me to think that the space is important there
Investigated further and this is what I found:

 perl -MO=Deparse -e 'sub a {}; my $r = \&a; sort $r (1,2)'           
+                                                
 sub a {

}
my $r = \&a;
sort $r 1, 2;
[download]

perl -MO=Deparse -e 'sub a {}; sort \&a (1,2)'
sub a {

}
sort \(&a(1, 2));
[download]