I still don't really understand your requirement, but here's a different approach. In my previous solution, the absolute offset of the start of the number field in the line had always to be the same if subsequent lines were to be rejected once a valid number line was detected. In this solution, the offset between the end of the  $toss_tag substring and the start of the number field must remain the same, wherever the toss tag happens to be in the string. So in the following string:

     +----------------------------------- any number of any characters
     |                                    before toss-tag field
     |
     |      +---------------------------- toss-tag field (defined
     |      |                             string, constant width)
     |      |
     |      |      +--------------------- any number of any non-digits
     |      |      |                      between end of toss-tag
     |      |      |                      and start of number field.
     |      |      |                      characters can vary after
     |      |      |                      first field seen, but width
     |      |      |                      cannot vary once set.
     |      |      |
     |      |      |         +----------- number field.  number cannot
     |      |      |         |            vary once set.
     |      |      |         |
     |      |      |         |      +---- any number of any characters
     |      |      |         |      |     after number field
     |      |      |         |      |
 /---+----\/+\/----+----\/---+---\/-+-----------------------\
'line 2 xx Foo NineChars 1234-12.3 other 123 first valid keep'
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Now that we know that the exact position of the key field does in fact change, that restriction is removed and all you need to do is find a suitably unique regex to capture it in the first place. Looks to me from

<DI:"def.def"><RD:Item><PS:Item>Campo_Ementa ###HERE GOES THE TEXT THA
+T I DONT WANT TO VERIFY[a-zA-Zetcetc]*###<HR>Campo_Identificacao(Recu
+rso Inominado nº <b>0702995-42.2017.8.01.0002</b>, 1ª Turma Recursal 
+dos Juizados Especiais/AC, Rel. Maria Rosinete dos Reis Silva. j. 11.
+07.2018 , Publ. 20.07.<FD:AnoJulg>2018</FD:AnoJulg>).<HR><PS:Identifi
+cacao>
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that it exists between "nº " and ", " and if that's the same for all of them then it is a simple switch:

use strict;
use warnings;
use utf8;

use Test::More tests => 1;

my @in = split (/\n/, <<EOT);
aaa nº 1234, xyz
bbb nº 1234, yzx
ccc nº 1234, yxz
ddd nº 2345, zyx
EOT

my @want = split (/\n/, <<EOT); 
aaa nº 1234, xyz
ddd nº 2345, zyx
EOT

my $number;
my %seen = ( '' => 1 );
my @out;

for my $line (@in) {
    if ($line =~ /nº ([\d]*),/) {
        $number = $1;
    } else {
        $number = '';
    }

    push @out, $line unless $seen{$number}++;
}

is_deeply (\@out, \@want);
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If that logic doesn't match your requirements, simply change the regex until it does. Obviously here, I'm just matching integers for the numbers and yours is more complex but the principle should still fit.