Which characters would I have to escape from a string, that I do not miss any ?

Use the \Q and \E quoting expressions to retain all those parts you wish to use as literal strings:

use strict;
use warnings;

use Test::More;

my @good = (
    'nnxx.yy2 = 333 abc',
    'nnxx.yy2 = 1 abc',
    'nnxx.yy2 = 2345 abc',
);

my @bad = (
    'foo',
    'nnxxayy2 = 1 abc',
    'nnxx.yy2 = 1a abc',
);

my $re = qr/^\Qnnxx.yy2 = \E\d+\Q abc\E$/;

plan tests => @good + @bad;

for my $str (@good) {
    like ($str, $re, "$str matched");
}
for my $str (@bad) {
    unlike ($str, $re, "$str not matched");
}
[download]