Ah!! it seems that nobody has noticed the wise hdb's advice!
This is a very important thing while coding, Know your data! or the other face of the coin: Bad data ruins your day

Compare the time and genius needed to modify all previous answer to NOT consider, let's say, AX_1 and  ZZ_2 as adjacent.

With plain code add a feaure to the code is normally a trivial task: just grab two parts from regular expression and one control more in the if loop.

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
[download]

L*