my $timeout = time + 10;

# method 1
#          --file still old--       --under 10 secs passed--
1 while (time - stat($file)[9] < 100 and $timeout > time);

# number 2 - very similar
sleep(1) while (time - stat($file)[9] < 100 and $timeout > time);

# then proceed
if (time - stat($file)[9] < 100) {
    # file updated, so do stuff
}
else {
    # child screwed up somewhere so log error
}