/^(.*)_(\d+)$/ and undef $h{$1}{$2} for <DATA>;

for(keys %h){
    for $x (sort keys %{$h{$_}}){
        if(exists $h{$_}{$x + 1} || exists $h{$_}{$x - 1}){
            print "$_ :  $x\n";
        }
    }
}

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