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use strict;
use warnings;
...
   } for keys %hash;

print Data::Dumper->Dumpxs( [ \ %hash ], [ qw{ *hash } ] );

%hash = (
          'preparation' => {
...
                               'frequency' => 1
                             }
        );

...
while ( <$inFH> )
...

delete $hash{ $_ }->{ seen } for keys %hash;
...