Ikegami is right, thats not working!

Better pass the list as a closed over array (here @x)

  DB<109> @a=  map { int rand 100 } 1..10
 => (96, 10, 99, 9, 43, 8, 20, 85, 42, 26)

  DB<110> sub minindex { 
             my @x=@_; 
             reduce { $x[$a] < $x[$b] ? $a : $b } 0 .. $#_ 
          }

  DB<111> print minindex @a
5
[download]

update

or even better like ikegami now suggested as ref to avoid overhead

  DB<114> @a=  map { int rand 100 } 1..10 => (22, 15, 27, 44, 70, 85, 
+74, 93, 96, 14)

  DB<115> sub minindex { 
             my $x=\@_; 
             reduce { $x->[$a] < $x->[$b] ? $a : $b } 0 .. $#_ 
          }

  DB<116> print minindex @a
9
[download]