This works for me:

use v5.12;
use warnings;
use HTML::Entities;
use Data::Dump qw/pp dd/;
use utf8;

my $sep = decode_entities('✶');

my $pat = "{${sep}Adjektive (Nominalflexion)~87$sep}";

my $text = join " foo \n", ($pat) x 2;


pp $text;

my $b = "Adjektive (Nominalflexion)~87";

my $c = "\Q{$sep$b$sep}\E";

my $r = "<div>some Text $b some other text</div>";

$text =~ s/$c/$r/sg;

pp $text;
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Tue Nov 23 13:06:24

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/nominal_flexion.pl 
"{\x{2736}Adjektive (Nominalflexion)~87\x{2736}} foo \n{\x{2736}Adjekt
+ive (Nominalflexion)~87\x{2736}}"
"<div>some Text Adjektive (Nominalflexion)~87 some other text</div> fo
+o \n<div>some Text Adjektive (Nominalflexion)~87 some other text</div
+>"

Compilation finished at Tue Nov 23 13:06:24
[download]