You can find out more about how "read" works by reading its documentation.
From there, you'll find out that the third parameter (your $record_length) is the number of bytes to read from the filehandle. As your template is set up to handle all of the data for one record in one go, you'll need to read one record's worth of data. That's 4 * (1 + 262792 + 1) bytes of data. Currently you're reading four bytes, and the template is looking for a lot more.
The documention for unpack says this:
If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: in some cases, the repeat count is decreased, or unpack() will produce null strings or zeroes, or terminate with an error. If the input string is longer than one described by the TEMPLATE, the rest is ignored.
 Actually, the number of _characters_ but let's assume single byte characters for the time being.
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