Notice the call to ->connect in the callback, overriding Sybase's? The callback mechanism provides a means of (conditional) overriding.

$sybase_drh->{Callbacks} = {
   selectrow_array => sub {
      # Protect against recursion when calling overriden method.
      return if $in_callback; local $in_callback = 1;

      my ($drh, $sql, $attrs, @bind_values) = @_;

      # Do something to $sql, $attrs and/or @bind_values here...

      # Tell DBI not to call original connect method
      undef $_;
      return $drh->selectrow_array($sql, $attrs, @bind_values);
   },
};
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Note that it would probably make more sense to override prepare and/or execute than convenience methods such as selectrow_array.

Update: Added code.