read first line from file_a
split the line into scalar variables ("machine" and "site")
open file_b
while reading a line at a time from file_b {
    if the line contains "machine", set $apply_edit to true
    elsif the line contains "Parent Submap: " and $apply_edit is true {
        replace the rest of this line with "site"
        reset $apply_edit to false
    }
    print the line
}