use Sort::Key::Maker iikeysort => qw(integer integer);
@sorted = iikeysort { /^(\d+)x(\d+)$/ } @a;
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update: and now even simpler:

use Sort::Key::Multi 'iikeysort';
@sorted = iikeysort { /^(\d+)x(\d+)$/ } @a;
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or if the chars between the two numbers can vary:

use Sort::Key::Multi 'isikeysort';
@sorted = isikeysort { /^(\d+)(\w+)(\d+)$/ } @a;
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