You are assuming that $files{'ENV.FILESOURCE.SOURCE.J2018059.N000001'} and $files{'ENV.FILESOURCE.SOURCE.J2018058.N000001'} point to unique hashs. but what if there was somewhere above this section a line that said:
$files{'ENV.FILESOURCE.SOURCE.J2018059.N000001'}=$files{'ENV.FILESOURC
+E.SOURCE.J2018058.N000001'};
while there would be an entry in $files for both names, they would both point to the same hash, so modifying either
$files{'ENV.FILESOURCE.SOURCE.J2018059.N000001'}{data} or
$files{'ENV.FILESOURCE.SOURCE.J2018058.N000001'}{data} would mean that the same end data location has been modified.
