-s FILE is often too big — not just when using DATA — since read is to be given a number of characters (as opposed to a number of bytes) when binmode isn't set to raw.

open(FILE, $0)
   or die("Unable to open $0: $!\n");

$read = read(FILE, $buf='', -s FILE);

print("Character mode:\n");
print("Bytes requested: ", -s FILE,      "\n");  # 580
print("Chars read:      ", $read,        "\n");  # 560
print("Chars read:      ", length($buf), "\n");  # 560

seek(FILE, 0, 0);
binmode(FILE);
print("\n");

$read = read(FILE, $buf='', -s FILE);

print("Bin mode:\n");
print("Bytes requested: ", -s FILE,      "\n");  # 580
print("Chars read:      ", $read,        "\n");  # 580
print("Chars read:      ", length($buf), "\n");  # 580
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But like you said, it's ok if it's too big.