Not sure if I understand your question ...

Like this  @input = glob '{A,C,T,G}' x 5 ?

For instance: (debugger demo)

  DB<4> x glob '{A,C,T,G}' x 2
0  'AA'
1  'AC'
2  'AT'
3  'AG'
4  'CA'
5  'CC'
6  'CT'
7  'CG'
8  'TA'
9  'TC'
10  'TT'
11  'TG'
12  'GA'
13  'GC'
14  'GT'
15  'GG'
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