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use strict;oruse warnings;(actually, it isstrictthat is most relevant here), there is no complaint about using%linesbefore it is defined. If run with strictures, there'd need to be a linemy %lines;before the loop. Note, too, that the hash is%lines; one element of the hash is referenced using the$lines{$_}notation.sortsolutions may be better for large amount of data (the OP was concerned about "storing the entire file in memory").sortwill perform an out-of-core sort if the data is larger than the available memory.