You've got it backwards. You need a byte in a pointer (pointer to a byte) not a pointer in a byte (which probably won't work on machines from the last couple of decades).

A string is already a sequence of bytes (ignoring stuff like unicode, etc), so theres no need to pack it as such.

Untested correction code:

my $blob = pack('LP', length($data), $data);
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There are a lot of issues dealing with packing/unpacking C structs. For details on these and ways around them, please see:

• perlpacktut (especially "Pointers for How to Use Them")
• c2ph
• Convert::Binary::C
• Inline::Struct