The j format is still architecture depending, as the others already showed. If your perl has 64bit ints (even if the poiters are just 32bit), does q (or Q) work for you? (for portability, do not forget < or >). That won't work on 32-bit-only perl builds.

$ perl -MDP -we'DHexDump pack "Q", 0x12345678'
0000  78 56 34 12 00 00 00 00                           xV4.....
$ perl -MDP -we'DHexDump pack "Q>", 0x12345678'
0000  00 00 00 00 12 34 56 78                           .....4Vx
$ perl -MDP -we'DHexDump pack "Q<", 0x12345678'
0000  78 56 34 12 00 00 00 00                           xV4.....
$ perl -V:uvsize
uvsize='8';

other perl

$ perl -V:uvsize
uvsize='4';
$ perl -wle'print length pack "Q", 3'
Invalid type 'Q' in pack at -e line 1.
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