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]

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]