GrandFather's solution may be shorten if a short one-liner is what you're after.

# reisinge
perl -E 'say [ sort { $ARGV[$b] <=> $ARGV[$a] } 0..$#ARGV ]->[0]' 42 1
+000 999 0 -1

# GrandFather
perl -E '$m=0; $ARGV[$m] > $ARGV[$_] or $m=$_ for 1..$#ARGV; say $m' 4
+2 1000 999 0 -1

# Shorten, similar performance
perl -E '$i=0; ($ARGV[$m] > $_ or $m=$i), $i++ for @ARGV; say $m' 42 1
+000 999 0 -1
[download]

Below, please find a benchmark script for testing against a large list.

use strict;
use warnings;
use List::Util 'shuffle';
use Time::HiRes 'time';

# Return index to biggest element.

sub reisinge {
    [ sort { $_[$b] <=> $_[$a] } 0 .. $#_ ]->[0];
}

sub GrandFather {
    my $idxMax = 0;
    $_[$idxMax] > $_[$_] or $idxMax = $_ for 1 .. $#_;
    $idxMax;
}

sub biggest_elm {
    my ($idxMax,$idx) = (0,0);
    ($_[$idxMax] > $_ or $idxMax = $idx), $idx++ for @_;
    $idxMax;
}

srand 0;

my @list = shuffle 1..4e5;

for my $code (qw( reisinge GrandFather biggest_elm )) {
    no strict 'refs';
    my ($start, $idx) = (time, $code->(@list));

    printf "Index %d, Seconds (%-11s): %0.03f\n",
        $idx, $code, time - $start;
}
[download]

Regards, Mario.

Edit: Added benchmark script.