What Mr Inscrutable almost tried to convey to you but couldn't be bothered to phrase in terms intelligible to non-aliens is:

You should not need to do anything to Perl strings in order to pass them to your DLL via Win32::API.

That is, you should not need to use pack or anything else. This should work:

use Win32::API;

my $dec = Win32::API->new('mydll.dll', 'OpenConnection', 'PPP', 'L') o
+r die $^E;

$return = $dec->Call( 'test', 'test', 'test' );

print $return;
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Assuming that there is a database called 'test', and a user called 'test', with 'test' as the password.

If that doesn't work, then you'll need to tell us how you know it didn't work.

Ie. Did you get an error message? If so, what was it? If not, what other indication do you have that it didn't work?

The proper way is

$return = $dec->Call($strDatabase, $strLoginUser, $strLoginPwd)
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You've even shown this works in (2).

If that doesn't work, your information about the function is wrong or your conclusion is wrong about it being related to your calling convention is wrong. Go verify those.