Not sure I'm completely following but it jumps out at me in your second block that the string in $b contains regular expression metacharacters (specifically parens) so that's prossibly the problem. Your "Adjektive (Nominalflexion)~87" is going to be treated as looking for the string "Adjektive" followed by a SPACE followed by the string "Nominalflexion" (which will be captured because of the parens) followed by "~87".

If you use \Q\E escapes to setup as $c = "\Q{$sep$b$sep}\E" that should appropriately escape the metacharacters and let them match literally.

Hi Fletch, thank you for your advice! I checked it with \Q...\E and also with (?: ...) which omits the capturing of the expressions in the parens, but nothing appears to help. I was wondering whether the tilde ~ produced trouble, but when quoted that should be the case. Best, Horst

Don't know what to tell you other than try providing a SSCCE that can actually be run. This below works as I expect it so you're doing something strange or (entirely possible) your problem statement's being misread.

(Also the <code> formatting is doing weird things but I actually have literal ✶ in my source and the output where it's being replaced with the entity below everywhere save the initialization of $wonky_char. Not sure what's the right way to get literal UTF8 chars in sample code using utf8.)

#!/usr/bin/env perl
use 5.034;

use HTML::Entities qw( decode_entities );
use utf8;

my $input = qq{{✶Adjektive (Nominalflexion)~87✶}};
my $wonky_char = decode_entities( q{✶} );

binmode( STDOUT, q{:utf8} );

say qq{\$input: $input};
say qq{\$wonky_char: $wonky_char};

my $to_match = "Adjektive (Nominalflexion)~87";
my $new_string =
  $input =~ s{\{$wonky_char(\Q$to_match\E)$wonky_char\}}{<div>I found 
+'$1'</div>}r;

say qq{\$new_string: $new_string};

my $cleaner_regex_sample =
  $input =~ s{ \{ $wonky_char (\Q$to_match\E) $wonky_char \} }{<div>Al
+so found '$1'</div>}rx;

say qq{cleaner: $cleaner_regex_sample};

exit 0;

__END__
$input: {&#10038;Adjektive (Nominalflexion)~87&#10038;}
$wonky_char: &#10038;
$new_string: <div>I found 'Adjektive (Nominalflexion)~87'</div>
cleaner: <div>Also found 'Adjektive (Nominalflexion)~87'</div>
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The cake is a lie.
The cake is a lie.
The cake is a lie.

Please show the output of

printf "%vX\n", $text;
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I bet your text doesn't actually contain ✶. Did you decode your inputs? You probably have it in its encoded form.

---

By the way,

my $sep = decode_entities('✶');
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is a complicated way of writing

my $sep = "\N{U+2736}";
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or

my $sep = "\x{2736}";
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or

use utf8;
my $sep = "✶";

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;
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-*- 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
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Encoding this variable together with the appropriate quoting suggested by the other monks solved the problem immediately

You should have made sure both strings were *decoded*.