Check DBI documentation for "LongReadLen". Something like the following in your connect statement might help:

my $dbh = DBI->connect("dbi:ODBC:$datasource", $username, $password
        , {LongReadLen => 1500, RaiseError => 1})
        || die "Error connecting: $!";
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HTH

Update: or it might not help, since you're using Win32::ODBC, not DBI!