why not use only grep?:

my @temp=("a_1","b_1","a_2","a_3","a_4","b_2","a_5","b_3","a_6","b_4")
+;

my @a_arr = grep { s/^a_(.*)/$1/} @temp;
my @b_arr = grep { s/^b_(.*)/$1/} @temp;
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PS: using @a and @b should be OK, but $a and $b have special meaning (in 'sort'), so I try to never use 'a' and 'b' as variable names.