use strict;
use warnings;
use Data::Dumper;
my @a = ("M94202", "M94150", "M94297", "M94150", "M94161", "M94161", "M94162");
my ( $n,  %h ) = -1;
$_ = $h{$_} //= $n+=2 for @a;
print Dumper \@a;

##</code><code>##

my ( $n,  %h ) = -1;
$_ = $h{$_} //= $n+=2 for @a;

##</code><code>##

$_=$h{$_}//=$}+=2-!$}for@a;