hi Dave,
thank you for your answer.
I have just tried your advice again - on another computer and without the "big file" - the patterns with the "second part" work now (I added two lines with text without dot in DATA below). Perhaps I made a typo a couple of hours earlier. I only modified the patterns with \b (s. below) since otherwise it matched 12-22a **This is trash*** too.
I'll try this tomorrow again with the "big file" and report here. Thank you very much again.
VE

use strict;
use warnings;
use 5.010;


my $pat1 = qr '(\b\d-\d\w{2}(?:\.\w+)?)';
my $pat2 = qr '(\b[A-Z]\d{2}(?:\.\d+)?)';



my $title;
while ( <DATA> ) {
  chomp;
  if ( /(.+)(\(.+\))$/  ) {      # Better regex for 'title' lines??
    $title = "$1$2;";
  }
  else {
    next unless /$pat1|$pat2/;
    my @items = grep length, split /$pat1|$pat2/;
    say $title, splice @items, 0, 2 while @items;
  }
  
}

__DATA__
Titel Text (A12-3)
3-123.7 Just another (small) text 3-123.8 Some more text
1-234 Text without dot 2-345 More text without dot
A35 Another text without dot
A12.34 Another item B56.78 Yet another item
Another Titel Text (B23-9)
Some trash here
12-22a **This is trash***
1-22a.b Just another text 
2-3cd.e Some more text
W12.34 Another item 
Z56.78 Yet another item Z56.78 And another!! Z56.7a And another!!!
Some trash
[download]