How about:

use if $] ge '5.016', feature => 'fc';
use if $] lt '5.016', 'Unicode::CaseFold' => 'fc';
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See Unicode::CaseFold. Otherwise you could do:

*case_insens = $] ge '5.016'
    ? sub { &CORE::fc($a) cmp &CORE::fc($b) }
    : sub { lc $a cmp lc $b };

@list = sort {case_insens()} @list;
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Update: Used &CORE::fc() instead of CORE::fc() so that it really does work on older Perls.