Well, the function is described at MSDN like this:

MCIERROR mciSendCommand(
    MCIDEVICEID IDDevice,
    UINT uMsg,
    DWORD_PTR fdwCommand,
    DWORD_PTR dwParam
);

So, naively choosing "long" ("N") for each parameter without looking at the headers to see exactly what things like "MCIDEVICEID" means, I would do something like this:

Win32::API->Import("winmm", "mciSendCommand", "NNNN", "N")
  or die $^E;
my $result = mciSendCommand($qux, $quux, $quuux, $quuuux);
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If you don't want to import the function into the current namespace, use my $coderef = Win32::API->new("winmm", "mciSendCommand", ...); instead and call like $coderef->(args).

Now, on my Vista 64-bit system, importing that function works and calling it with all-zero parameters does not result in a crash, so we're on the right track. (Though, those DWORD_PTR arguments may need to be of type "P" -- test and see what works).

Hope that helps.