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