... no value has ever been assigned to the variable $sequence.

No, it means $sequence is undef at that point, which was probably assigned to it by the statement the OP is having trouble with, $sequence=$fastas{$gene_name};. ($sequence could very well have held a value before then.)

... when you try to use $sequence as a hash key...

They're not, you probably mean $gene_name.

In reply to Re^4: hash access with a variable by Anonymous Monk
in thread hash access with a variable by LostWeekender

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