my $result="";
my $re = qr/
            (.*?)   # Everything before the first tag
            $this->{mask_start}
            \Q$key\E(.*?)
            $this->{mask_end}
            (.*?)
            $this->{mask_start_close}
            (?=(.*))  # capture the rest, to get end of template
           /x;

my $trailingText;

while($this->{template} =~ /$re/g) {
    $result.=$1;  # add the stuff between tags to results
    my $params = $this->mask_block_params($2, $3);
    my $html = &$callback($key, $params);
    $result.=$html;

    $trailingText=$4;
}

$result.=$trailingText;

# $result should be what you're looking for.  I think :)