I don't know if the Sybase driver supports it, but flaky servers suggest the DBI $dbh->ping method. You can open a handle to each server, say $dbx and $dby, and use the trinary op to select my $sth = ($dbx->ping ? $dbx : $dby)->prepare('select 1+1'); and so on. That will get awkward fast.

Another approach is to first try to connect to x. If that fails, connect to y. That isn't what you want for a long running process, but for a one-shot script it is probably ok (you wanted tolerant, not immune, right?)

my $dbh = connect($x_string, $x_nam, $x_pass)
        || connect($y_string, $y_nam, $y_pass)
        or die 'No db!';
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In any case, you should be meticulous about error checking with each DBI call.