Everything you need is already there:

#!/usr/bin/env perl

use strict;
use warnings;
use feature qw(say);
use List::Util qw(max shuffle);
use List::MoreUtils qw(onlyidx);
use Data::Dump;

my @array = shuffle (1..10);

dd \@array;

my $max =  max @array;

say $max;
say onlyidx {$_== $max} @array;

__END__


Karls-Mac-mini:Desktop karl$ ./watermark.pl 
[7, 10, 9, 8, 2, 6, 4, 3, 5, 1]
10
1
[download]

I just cannibalized some fragments i found on my desktop in a hurry. But as far as i remember List::Util and List::MoreUtils come with implementations in pure Perl which might be good for further studies.

Best regards, Karl