There is a bug in the prototype definition. The quickest solution is to patch the Win32::API module by 'voiding' the custom prototype.

...
my $f = Win32::API->new("winmm", "long mciSendString(LPSTR a, LPSTR b,
+ long c, long d)");

delete $f->{proto};
delete $f->{intypes};
...
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This will force Win32::API to behave the same as

Win32::API->new("winmm", "PPNN", "N");
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It is often useful to turn on the internal debugging of Win32::API to work out what is the prototype parser doing. Turn on debugging with $Win32::API::DEBUG = 1;. This shows that in your first prototype, the char * was interpreted as char. Win32::API was not doing a good job at parsing char *. (grin...)