This is a perl one-liner doing just what you want:
$ echo 'ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
> ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA' | perl -nE 'say substr($_,
+ 10, 1);'
C
C
C
C
C
C
C
C
C
C
C
C
C
Check, though, that 10 is the right second parameter for substr, you may have to change it depending on which character you want exactly.