my $bytes = 0x123456;
print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack("L",0x12345678))), "\n";
print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack("L",$bytes))), "\n";

#output:
#0x78 0x56 0x34 0x12
#0x56 0x34 0x12 00