if @myfile is tied file array: (...) $size=@myfile; #this dies and gives "out of memory" error

This issue seems to be directly addressed in the docs to Tie::File (in the caveats):

Note that accessing the length of the array via $x = scalar @tied_file accesses all records and stores their offsets.

Did you try, for example:

print $myfile[-1];

In reply to Re: Large text files into arrays, accessing final elements by Not_a_Number
in thread Large text files into arrays, accessing final elements by isabella423

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