in reply to How to easily convert the pack function from little endian to big endian

Yes:

    read EP, $in, 2;
    $rec_len = unpack ("S>", $in);

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But whether that is the right output will depend upon what you got sent.

This would also produce the same output on either type of machine:

    read EP, $in, 2;
    $rec_len = unpack ("S<", $in);

[download]

But which of those would produce the correct output, depend entirely on whether the data was packed in little or big-endian order to start with.

And that cannot be determined by the receiver; it can only be assumed.

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