As to the efficiency of your data structure in memory, I'll leave that to others. Being as I forget from time to time and am stung by the lack of defined in the below, here is a basic while loop for reading line by line.

The defined call ensures your loop does not break prematurely should the line evaluate to a false value.

use autodie;

open(my $fh, '<', $file);

while ( defined (my $line = <$fh>) )
{
   chomp($line);
   # do stuff
}
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