You have a string with a regular pattern of \n between the tokens. You want the first \n separated token and then the next 3 \n separated tokens. Maybe I have misunderstood the problem, but I would use split and slice for this job.

#!/usr/bin/perl -w
use strict;

my $string = "P5§2\nAAA\nBBB\nCCC\nP6§21\blah\nmore\n"; 

my ($first,@tokens) = (split(/\n/,$string))[0..3];

print "$first\n";
print @tokens,"\n";

__END__
Prints:
P5§2
AAABBBCCC


#Update: if you want the tokens in one $var, say $stuff, then
#my $stuff = join("",@tokens);
#print "$stuff\n";  # also prints: AAABBBCCC
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