I usually make those the first two non-comment non-blank lines in the file; and I put use strict; first as a matter of aesthetic taste — that both puts them in alphabetical order and gives a nice visual lead towards the right, where the later use statements are likely to end, since most module names are longer than warnings.

Example:

use strict;
use warnings;

use Storable;
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Some very useful modules break this pattern:

use strict;
use warnings;

use DBI;

use Tk;
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