I'd expect it four times when using &foo, and I also get it. B::Deparse to help you understand:

perl -MO=Deparse -e 'use strict;
use warnings;
sub foo ($) {
    my $x = shift;
    print "foo gets '$x'\n";
    return ($x+2, 33);
}
my @a = foo(42);   
print "a=@a\n";   
@a = sort(foo(42)); 
print "a=@a\n";  
@a = (sort foo(42));  
print "a=@a\n";    
@a = sort foo(42);  
print "a=@a\n";
'
sub foo ($) {
    use warnings;
    use strict 'refs';
    my $x = shift @_;
    print "foo gets \n";
    return $x + 2, 33;
}
use warnings;
use strict 'refs';
my(@a) = foo(42);
print "a=@a\n";
@a = sort(foo(42));
print "a=@a\n";
@a = (sort foo 42);
print "a=@a\n";
@a = (sort foo 42);
print "a=@a\n";
-e syntax OK
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In other words, sort foo(42) is understood as "sort the list containing 42 by sorting function foo".