Nothing's intrinsically wrong with it, so far as that goes. However, what ends up happening is that you have a global variable in one place. This could lead to a slippery slope. I'm not saying that someone of your experience will do so, but it could very well happen.

Instead of that syntax, I would recommend one of the following:

package MyClass;
@ISA = qw(Whatever);

use strict;

-----

use strict;
use vars '@ISA';
package MyClass;
@ISA = qw(Whatever);

-----

use strict;
use 5.6.0;
package MyClass;
our @ISA = qw(Whatever);
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The first two still use globals, and are for pre-5.6.0 or before. If you require that 5.6.0 or higher be used, there's absolutely not reason not to use our. None.