my $cell_name = "xyz";
while (<DATA>) {
    if (/STARTING_PATTERN\s+(.*)/) {
        $string = $_;
        $sub    = quotemeta $1;
        $string =~ s/$sub/$cell_name/;
        print $string;
    }
}

__DATA__
STARTING_PATTERN RRR

##</code><code>##

STARTING_PATTERN xyz