in reply to reference of hash

I see a couple of problems:

  1. Inside flow_count, you say $HASH_OF_FLOW->{'$key'} = 'M2'. By surrounding $key with single quotes, you're getting the string '$key' rather than the value in the variable $key. Drop those quotes.
  2. In your foreach loop, you're not dereferencing your hashrefs. For example, where you say $FTFM_HASH_OF_FLOW{$key}, you should be saying $FTFM_HASH_OF_FLOW->{$key}.
  3. You're not using strict and warnings -- if you were, you'd have been scolded for #2, above.

As an aside, you may want to read up on How (Not) To Ask A Question; it will advise you on how to get better responses. For example, providing the text of the error you're receiving would have been useful.

You might also want to take a look at Data::Dumper; if you'd used that to inspect the contents of $HASH_OF_FLOW, you'd have very quickly seen problem #1.

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“A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.” Herm Albright
I haven't found a problem yet that can't be solved by a well-placed trebuchet