It's almost a one-liner :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1148176

use strict;
use warnings;

my %used;

print grep /_/ && !$used{$_}++, (join '', <DATA>) =~
  /^(.*?(\d+)\n)(?=(.*?(\d+)\n))(??{1 != abs($2 - $4)&&'(*F)'})/gm;

__DATA__
2L_33
2L_34
3L_45
3L_87
X_202
X_203
X_204
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