$pos = 0;
while ($m = shift @markers) {
    # locate the beginning of the link
    last if (($pos = index $htmlfile, '<a', $pos) < 0);
    # ...then the start of the link's href
    last if (($pos = index $htmlfile, 'href="', $pos) < 0);
    # skip past the first "
    $pos += 6;
    # ...then the end of the quoted href target
    last if (($pos = index $htmlfile, '"', $pos) < 0);
    substr($htmlfile, $pos, 0) = $m;
}