You asked sysread to read 12 bytes, not 12 fields. unpack("I") will only decode one 32-bit value from the beginning of the buffer passed to it.

Do you have a C type structure definition or some other description for the data you are trying to parse?

My Pack/Unpack Tutorial might be helpful to you.

---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
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