my $dir = 'C:/test/';
opendir DIR, $dir or die $!;
while (my $file = readdir DIR ) {
    next unless $file =~ m/\.html?/;
    my @stat = stat $dir.$file;
    $mod_time = scalar localtime $stat[9];
    # do an inplace edit becaue it is less typing...
    `perl -pi.bak -e "s!(<MODTIME>[^<]*</MODTIME>)?</BODY>!<MODTIME>$mod_time</MODTIME></BODY>!i" $dir$file`;
}
closedir DIR;