my @sizes = qw/6XS 5XS 6XL L S xL m 3xs/;

my $sort_sizes_sub = sub {
  my $uc_a = uc($a);
  my $uc_b = uc($b);
  my @sizes_sorted = qw/6XS 5XS 4XS 3XS 2XS XS S M L XL XXL 2XL 3XL 4X
+L 5XL 6XL/;
  return 0 if $uc_a eq $uc_b;
  foreach my $s (@sizes_sorted) {
    if ($s eq $uc_a) {
      return -1;
    }
    if ($s eq $uc_b) {
      return 1;
    }
  }
};

print join(' ', @sizes). "\n";
print join(' ', sort $sort_sizes_sub @sizes). "\n";
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Result:

6XS 5XS 6XL L S xL m 3xs
6XS 5XS 3xs S m L xL 6XL
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