I don't know what could be happening. Either size or blksize is undefined (if your line number is correct), and coming into the subroutine there's this line:

my $size = shift || croak 'read($buf,$size,[$timeout])';
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So the script should die if size is undefined, and blksize is set when you create the ftp object, and you don't seem to be changing blksize anywhere. What version of Net::FTP do you have? Though it seems unnecessary on such a short script, my only advice is use strict and warnings, at least on the larger script this script comes from, and maybe step through with the perl perldebugger.