With the help of Windows' ODBC Data Source Administrator, I have configured a system DSN (named $myDSN)

Since you've already created the DSN via Windows ODBC Data Source Administrator, you just need to use that $myDSN var in your connection string. The ODBC driver will use that name to lookup the connection details (i.e., the specific driver, server name, port, and db name)

my $myDSN = 'data_source_name';
my $dbh = DBI->connect("DBI:ODBC:$myDSN", 'user', 'password',
                       { RaiseError => 1}) or die DBI->errstr;
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