Well its rather more philosophical question, than a real life situation... Why I would like to "replace bits of a tainted string with other bits and continue to consider it tainted"? Well simple example

sub get_file_and_args {
    my ($path, ) = @_;
    my (@info, );
    while (1) {
        last if ((@info = stat ($path)));
        last if ($path !~ s{^(/.+)(/+.*)}{$1});
        push (@args, $2);
    }
    
    return ($path, \@args);
}
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where $path = '/path/to/a///file/with///few//args'; and is tainted.

Later on I want to force checking both $path and @args values (by tainting them) because I cant really be sure who and how is using them. Im not discussing if the same result can be achieved in any other 'better' or elegant way because the answer is 'yes, of course!' - I just want to show that there might be a reason "to replace bits of a tainted string with other bits and continue to consider it tainted".