use List::Util qw( reduce );

sub min_idx {
   my ($ar) = @_;
   return reduce { $ar->[$b] > $ar->[$a] ? $b : $a } 0..$#$ar;
}

##</code><code>##

sub min_idx {
   my ($ar) = @_;
   return undef if !@$ar;

   my $idx = 0;
   for (1..$#$ar) { $idx = $_ if $arr->[$_] > $arr->[$idx]; }
   return $idx;
}

##</code><code>##

my $min_idx = min_index(\@a);