Thank you for your code and hint. Updated, see downwards

I tried the code as is, and I get a lot of

Use of uninitialized value in concatenation (.) or string at ./data li
+ne 9, <DATA> chunk <number of data line>.
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and a lot of empty hits

cn: 
gid: 
members:
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mixed up data

cn: username
gid: 102
members:
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but also hits, which are right.
This might be lead from dn: being inside irrelevant data blocks.

How to face a block, and only check it out, if it starts with "dn: cn" at the very first line? This would disregard irrelevant data blocks.

Update:

if(/^dn: cn/) {  # skip irrelevant paragraphs
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does exactly what I need. Paragraph mode is the one!
supercool