use strict;
use warnings;
use Data::Dumper;

my @array = ("M94202", "M94150", "M94297", "M94150", "M94161", "M94161", "M94162");

my %seen;
my $z = 1;
foreach my $item (@array) {
    if (exists $seen{$item}) {
        $item = $seen{$item};
    }
    else {
        $seen{$item} = $z;
        $item = $z;
        $z += 2;
    }
}
print Dumper \@array;
print "\n";