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a1_1[1] a1_1[2] a1_1[10] a1_2[1] a1_2[2] a1_2[10] a1_10[1] a1_10[2] a1
+_10[10]
a2_1[1] a2_1[2] a2_1[10] a2_2[1] a2_2[2] a2_2[10] a2_10[1] a2_10[2] a2
+_10[10]
...
b1_1[1] b1_1[2] b1_1[10] b1_2[1] b1_2[2] b1_2[10] b1_10[1] b1_10[2] b1
+_10[10]
b2_1[1] b2_1[2] b2_1[10] b2_2[1] b2_2[2] b2_2[10] b2_10[1] b2_10[2] b2
+_10[10]
b10_1[1] b10_1[2] b10_1[10] b10_2[1] b10_2[2] b10_2[10] b10_10[1] b10_
+10[2] b10_10[10]

my @new_list = sort {
 
...
 ($a =~ /\[(\d+)/)[0] <=> ($b =~ /\[(\d+)/)[0] 

} @split_list;