Just for you to know another trap of hashes of file handles, if you want to write to a file handle in a hash, you usually have to enclose the hash element between curly braces. For example:

print {FH_hash{$foo}} $line_out;
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I tried to see if enclosing the file handle within curlies for reading with the diamond <> operator might work:

while (<{FH_hash{$foo}}>) { #...
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but that does not help. Using the readline function is really the right solution.