# always use stric and warnings (till the moment you know when is safe disabling them)
use strict;
use warnings;
# we use an array to grab DATA. array preserves order, if order in the output is needed
my @arr;

# <> is something like an iterator:
# $next_line = <DATA> retrieve next line
# for <DATA> process all lines
# we chomp all lines to remove \n at the end and then we push the @arr with the line
chomp $_ and push @arr,$_ for <DATA>;

# hashes provides uniqueness of keys, and we need uniqueness because...
my %adj;

# .. in the loop from 0 to the last index of @arr
# (pay attention when using $#arr: @arr in scalar context return num of elements,
# while $#arr is the last index of the array starting from 0
# so scalar @arr == $#arr + 1)
# in the loop we process two value at time (sliding window?) checking if the
# numerical part is adjacent to the next element's numerical part
for (0..$#arr){
    # exit condition go EVERYTIME at the beginning of loops
    # so we will exit the loop if is the last element (yet processed previously)
    last if $_ == $#arr;
    
    # grab the numerical part of interest
    # $1 is what inside the first matched () group. (capturing parentheses)
    my $cur_num = $1 if $arr[$_] =~/\d*[A-Z]_(\d+)$/;
    my $next_num = $1 if $arr[$_ + 1] =~/\d*[A-Z]_(\d+)$/;
    
    # if current is adjacent to next
    if ($cur_num == $next_num - 1){
                # we populate the hash with nevermind values
                $adj{$arr[$_]} = undef;
                $adj{$arr[$_ + 1]} = undef;
                
                # if we had used $adj{$arr[$_]}++ (autoincrement)
                # you would notice the X_203 with value of 2
                # because is inserted twice: as next_num while processing X_202
                # and as current_num while X_204
    }
}

# if the order of the data must be preserved we still have the array:
# if the data was alphabetically ordered would be simpler (and the array unuseful)
# simple as print "$_\n" for sor keys %adj
foreach (@arr){
  print "$_\n" if exists $adj{ $_ };
}
__DATA__
2L_33
2L_34
3L_45
3L_87
X_202
X_203
X_204