I had to take a stab at this. I am not proud of having to insert the final loop to filter out repeats, but maybe someone else can fix that. ;)

This solution splits each data element by underscore and sorts by both the left and right, which hopefully will line up any stray adjacent elements and prevent having to look behind.

use strict;
use warnings;

my %ordered;
for (<DATA>) {
    chomp;
    my ($left,$right) = split /_/, $_, 2;
    push @{ $ordered{$left} }, $right;
}

my @matches;
for my $key (sort keys %ordered) {

    my $last = 0;
    for (sort @{ $ordered{$key} } ) {
        push @matches, "${key}_$last","${key}_$_" if $_ - $last == 1;
        $last = $_;
    }
}

my %seen;
for (@matches) {
    print "$_\n" unless $seen{$_}++;
}

__DATA__
X_203
2L_33
3L_45
X_202
2L_34
X_204
2L_32
3L_87
3L_88
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)