Separate from the question of handling UTF-8 source code, here are some comments on the regexes.

... use "§" as delimiter ... because I use REGEX on written text and I've found, that nearly any character including brackets will be able to be included in the text ...

But the  s/// m// delimiter will not clash with any character in the "bound" text variable nor in an interpolated  qr// regex object or plain string:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $text = 'foo/bar/baz/boff zip/zit/zot/zap';
 print qq{'$text'};
 ;;
 my $regex_object = qr{ /bar/baz/ }xms;
 my $plain_string =    '/zit/zot/';
 ;;
 $text =~ s/ $regex_object | $plain_string /OTHER/xmsg;
 print qq{'$text'};
"
'foo/bar/baz/boff zip/zit/zot/zap'
'fooOTHERboff zipOTHERzap'
[download]

(However, note that interpolation of plain strings is problematic if they may contain regex metacharacters; for this, see quotemeta and the  \Q...\E interpolation modifiers.)

The use of  () {} [] <> as balanced regex delimiters is useful because balanced delimiters | delimiter characters within the regex pattern are handled properly (within reason; character classes present exceptions, but | unescaped delimiter characters within the regex pattern must always be strictly balanced, so  [{}] would have worked in the example below):

c:\@Work\Perl\monks>perl -wMstrict -le
"my $text = 'foo {bar} baz { whiz } boff';
 print qq{A: '$text'};
 ;;
 $text =~ s{ { \s* \w+ \s* } }{OTHER}xmsg;
 print qq{   '$text'};
 ;;
 $text = 'abc {tuvw} de { xyz } fghi';
 print qq{B: '$text'};
 ;;
 $text =~ s{ [\}\{] \s* \w+ \s* [\}\{] }{OTHER}xmsg;
 print qq{   '$text'};
"
A: 'foo {bar} baz { whiz } boff'
   'foo OTHER baz OTHER boff'
B: 'abc {tuvw} de { xyz } fghi'
   'abc OTHER de OTHER fghi'
[download]

do { $foundstring =~ s§(<a |\[)([^<>\"]*)(<span class=\"foundterm\">)~
+~([^~]+)~~(</span>)§$1$2$4§igs;    } while $foundstring =~ m§(<a |\[)
+([^<>\"]*)(<span class=\"foundterm\">)~~([^~]+)~~(</span>)§is;
[download]

Doing a substitution that is dependent on a separate, identical  m// match in this way is redundant because the  s/// replacement will only occur if its own match is successful, and the  /g modifier will cause all matches to be replaced:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $text = '123 abc 456 de 789 fghi 321';
 print qq{A:    '$text'};
 ;;
 do {  printf 'running s/// -> ';
       $text =~ s{ [a-z]+ }{OTHER}xmsg;
       print qq{'$text'};
       }
 while $text =~ m{ [a-z]+ }xms;
 print qq{done: '$text'};
 ;;
 $text = '123 rs 456 tuvw 789 xyz 321';
 print qq{B: '$text'};
 ;;
 $text =~ s{ [a-z]+ }{OTHER}xmsg;
 print qq{   '$text'};
"
A:    '123 abc 456 de 789 fghi 321'
running s/// -> '123 OTHER 456 OTHER 789 OTHER 321'
done: '123 OTHER 456 OTHER 789 OTHER 321'
B: '123 rs 456 tuvw 789 xyz 321'
   '123 OTHER 456 OTHER 789 OTHER 321'
[download]

In case A, the while-loop and substitution only run once because the  /g modifier of the  s/// causes anything that could match to be replaced. In case B, the same result is achieved with no separate  m// match.

Update: One sometimes sees something like
    my $match = qr{ ... }xms;
    $string =~ s{ $match }{replace}xms if $string =~ m{ $match }xms;
as a variation on this theme. Again, the substitution will only occur if the  $match pattern matches, so the separate  m// on the same pattern is redundant.