use strict;
use warnings;

my @arr;

chomp $_ and push @arr,$_ for <DATA>;

my %adj;

for (0..$#arr){
    last if $_ == $#arr;
    my ($cur_num,$cur_code)= ($2,$1) if $arr[$_] =~/(\d*[A-Z])_(\d+)$/;
    my ($next_num,$next_code)= ($2,$1) if $arr[$_ + 1] =~/(\d*[A-Z])_(\d+)$/;
    
    if (($cur_num == $next_num - 1) and ($cur_code eq $next_code)  ){
                $adj{$arr[$_]} = undef;
                $adj{$arr[$_ + 1]} = undef;
    }
}

foreach (@arr){
  print "$_\n" if exists $adj{ $_ };
}
__DATA__
AX_1
ZZ_2
2L_33
2L_34
3L_45
3L_87
X_202
X_203
X_204