Modified regex to accomodate bil:

#!/usr/bin/perl -w

use strict;

while (<DATA>) {
    s/(.*ing)((?!bob|fred|bill(?!ess))\w+?)(more|less)/$1HIT$3/;
    print $_;
}

__DATA__

somestringbobmoremore
somestringfredmoremore
somestringbillmoremore
somestringtedmore
somestringbilless
[download]

Prints:

somestringbobmoremore
somestringfredmoremore
somestringbillmoremore
somestringHITmore
somestringHITless
[download]

Thanks to everyone for working solutions provided. To further refine the OP, as I tried to simplify for the sake of clarity : Consider :

#!/usr/bin/perl -w

use strict;

while ( <DATA> ) {


        s/ angle brackets between element pairs / instead make square 
+bracketed /


        print $_ . "\n";


}

__DATA__

<sometag> my data here </sometag>
<anothertag> further text </anothertag>
<furthertag> not good as contains <this> in angle brackets </furtherta
+g>
<byetag> another possibility is <more> <than> one <angle pairs> in her
+e </byetag>
[download]

As I can no longer specify (more|less) as fixed patterns ( as per ikegami hunch ) Im again struggling to apply what is demonstrated to my actual data... Required output :

<sometag> my data here </sometag>
<anothertag> further text </anothertag>
<furthertag> not good as contains [this] in angle brackets </furtherta
+g>
<byetag> another possibility is [more] [than] one [angle pairs] in her
+e </byetag>
[download]

That looks like XML. I'd seriously consider using XML::Twig!

If you need some help show us some representative data and what you actually want to extract

---

DWIM is Perl's answer to Gödel

This violates your "no code" premise, but if your tags are all on one line, this seems reasonably robust and should be fairly efficient.

#! perl -sw
use strict;

while( <DATA> ) {
    s[^(<([^>]+?)>)(.*)(</\2>)]{
        (my $x = $3) =~ tr[<>][[]];
        "$1$x$4";
    }e;
    print;
}


__DATA__
<sometag> my data here </sometag>
<anothertag> further text </anothertag>
<furthertag> not good as contains <this> in angle brackets </furtherta
+g>
<byetag> another possibility is <more> <than> one <angle pairs> in her
+e </byetag>
<a tag with spaces> and content containing a false </a tag with spaces
+> and a real </a tag with spaces>
[download]

Produces

C:\Perl\test>junk2
<sometag> my data here </sometag>
<anothertag> further text </anothertag>
<furthertag> not good as contains [this] in angle brackets </furtherta
+g>
<byetag> another possibility is [more] [than] one [angle pairs] in her
+e </byetag>
<a tag with spaces> and content containing a false [/a tag with spaces
+] and a real </a tag with spaces>
[download]

Of course, it doesn't attempt to deal with attributes, or multi-line elements or nested tags or any of that good stuff.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

It is just what OP asked for. If OP wanted to match more or less then:

does the trick.

Note that OP says:

Im getting stuck specifying 'not' - OK with a caracter class, ( ie ^some chars ) - but not when using parenthesis to define multiple 'words'

Nope, doesn't work.

#!/usr/bin/perl -w

use strict;

while (<DATA>) {
    s/(.*ing)((?!bob|fred|bill)\w+?)(more|less)/$1HIT$3/;
    print $_;
}

__DATA__
somestringbilless
[download]

outputs

somestringbilless
[download]

and not

somestringHITless
[download]

which is actually correct (bill is rejected remember). Try somestringtedless instead (ted is not rejected - lucky ted)

---

DWIM is Perl's answer to Gödel