You just need to ensure that the character encoding is correctly treated in your source. eg:

$ perl -Mutf8 -e 'print "Yes\n" if "a" =~ m§a§;'
Yes
$

See the docs for the utf8 pragma for more info on that. If your source is in some other encoding, now might be an excellent time to switch.

Hello hippo,

It's not what I expected, but thanks for the hint. For some circumstances this would take me little bit more effort. But may be I will find some character I can use in the short way.

regards and have a nice day

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]

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]

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.

Hello AnomalousMonk,

great answer, helpful hints. That is helping others in best manner. Thanks very much for it.

I had some ideas why I took this redundancy. But it might become too long here to discuss. One was about uncertainty on nested matches and the wish of doing it one after the other. About the redundancy I was aware.

So thanks again and have a nice day