The reason has nothing to do with pack or unpack, it is the same reason that printing a hex constant doesn't print exactly what you see.

print 0x428c0000;

1116471296
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Perl automatically converts hex values (as indicated by 0x...) to it internal (binary) representation when parsing the source code. It becomes just another perl number.

Whenever perl prints out a number, it prints the ascii representation of it's decimal value, unless you specifically state that you want it printed in some other form. Ie. using printf or sprintf or similar.