Note that hashgrep returns both keys and values, so you should have a hash on the left hand side of the assignment. If you are just interested in the keys, you can use normal grep with keys:

my @match = grep { $SOCCalc >= $_ } keys %$dischargelookuphash;
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If you want to frequently extract ranges from a structure, hash is probably not the best tool for the job. A heap or a tree might serve you better.