From a file handle:

local $/ = \128;
while (<$fh>) {
   ... $_ contains 128 bytes ...
}
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From a string:

while (length($str)) {
   my $rec = substr($str, 0, 128, '');
   ... $rec contains 128 bytes ...
}
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Note that offsets should be 0-127, 128-255, etc.