I was trying to learn prototypes, so decided to make a reduce function as follows. 
=usage
    reduce \&f @x;
eg.
    reduce {$a+$b} @x;        # sum @x
    reduce {$a*$b) 1..7        # factorial(7) = 7!
=cut

sub reduce (&@)
{ my $f = shift;
  local ($a, $b) = shift;
  $a=&$f while $b=shift;
  return $a;
}

my $x = reduce { $a*$b } 1..7;
print $x;

[download]
It works. My question is how can I get it to have semantics exactly like sort?
Update: Since there seems to be some confusion as to what I meant by this question, let me elaborate from the documentation about sort:

If the subroutine's prototype is ($$), the elements to be compared are passed by reference in @_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b.

SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use.

moritz and duelafn seem to have addressed most of the issues.

In reply to How to get sort-like semantics? by b4swine

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