This is a good point and the thing I am trying to find out.

The syntax of DBI shows that my syntax is correct.

If I try the one you points out, the connection always fails with (tried it with and without empty space after comma):

DBI connect('Driver={SQL Server};Server=RC4FA9\SQLEXPRESS,1433;Databas
+e=test;UID=;PWD=','HASH(0xc2d3c50)',...) failed: [Microsoft][ODBC SQL
+ Server Driver][DBMSLPCN]SQL Server does not exist or access denied. 
+(SQL-08001) [state was 08001 now 01000]
[Microsoft][ODBC SQL Server Driver][DBMSLPCN]ConnectionOpen (Connect()
+). (SQL-01000) at ....
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The second parameter to DBI->connect is the username and the third parameter is the password. You are passing in the options hash:

DBI->connect(
    "dbi:ODBC:Driver={$ODBCdriver};Server=$SQLserver;Database=$SQLdata
+base;UID=$SQLuser;PWD=$SQLpassword;Port=$SQLport",
    { PrintError => 0,
      RaiseError => 1,
      AutoCommit => $AutoCommit,
      FetchHashKeyName => 'NAME_lc'
    },
);
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I would split that up, and construct the values separately to keep things somewhat understandable:

my $dsn = "dbi:ODBC:Driver={$ODBCdriver};Server=$SQLserver;Database=$S
+QLdatabase,$SQLport;UID=$SQLuser;PWD=$SQLpassword; ...";
my $dbi_options =     { PrintError => 0,
      RaiseError => 1,
      AutoCommit => $AutoCommit,
      FetchHashKeyName => 'NAME_lc'
    };
my $dbh = DBI->connect( $dsn, undef, undef, $dbi_options );
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This is a good suggestion and makes the call more readable. I implemented it. However, it does not solve the issue. Calling:

my $dsn = "dbi:ODBC:Driver={$ODBCdriver};Server=$SQLserver,$SQLport;Da
+tabase=$SQLdatabase;UID=$SQLuser;PWD=$SQLpassword; ...";
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Note that I moved ',$SQLport' to Server. This syntax never connects. This is probably because it is not the syntax expected by Perl DBI (all examples on Perlmonks too also use '; Port=1433' which instead always connects no matter the port I use -> this indicates - I guess - that the parameter is not used. To be honest, I have no ideas.

that 'HASH(0xc2d3c50)' does not look good to me, perhaps there is a problem constructing your dsn?