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;
-*- 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