What you have there is fine. Although, if you don't need it for anything else, you can eliminate @hierarchy and shorten the code a bit. Don't forget warnings too.

#!/usr/bin/env perl 
use strict;
use warnings;

my $order = 0;
my %hierarchy;

$hierarchy{$_} = $order++ for @ARGV;

while( my( $key, $value ) = each %hierarchy ){
    print "$key: $value\n";
}
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