#
# rename this replace.pl
#! /usr/bin/perl -w
# perl replace.pl cipherLetter plainLetter < cipherText [ > updatedCipher ]

$cipher = shift;  # shift first arg into a var
$plain  = shift;  # shift second arg into a var
                  # were in array @ARGV

while ($line = <>) {
  $line =~ s/$cipher /$plain /g; 
  print $line;
}

bash script
#!/bin/sh

echo -e "\n\n\n"
./replace.pl 31532 t < Cipher4.txt > tmp1
./replace.pl 5493 h < tmp1 > tmp2
./replace.pl 4028 e < tmp2 > tmp1
./replace.pl 9023 o < tmp1 > tmp2
./replace.pl 10375 s < tmp2 > tmp1
./replace.pl 77712 t < tmp1 > tmp2
./replace.pl 64091 t < tmp2 > tmp1
./replace.pl 7493 u < tmp1 > tmp2
./replace.pl 432 w < tmp2 > tmp1
./replace.pl 11052 g < tmp1 > tmp2
./replace.pl 2333910 y < tmp2 > tmp1
./replace.pl 31533 a < tmp1 > tmp2
./replace.pl 34082 n < tmp2 > tmp1
./replace.pl 27331 d < tmp1 > tmp2
./replace.pl 456 r < tmp2 > tmp1
./replace.pl 35132 e < tmp1 > tmp2
./replace.pl 23492 l < tmp2 > tmp1
./replace.pl 10333 e < tmp1 > tmp2
./replace.pl 30482 m < tmp2 > tmp1
./replace.pl 20493 i < tmp1 > tmp2
./replace.pl 732011 c < tmp2 > tmp1
./replace.pl 64901 e < tmp1 > tmp2
./replace.pl 4082 a < tmp2 > tmp1
./replace.pl 250111 f < tmp1 > tmp2
./replace.pl 29310 v < tmp2 > tmp1
./replace.pl 4563 a < tmp1 > tmp2
./replace.pl 100573 k < tmp2 > tmp1
./replace.pl 30 p < tmp1 > tmp2
./replace.pl 93 x < tmp2 > tmp1
./replace.pl 3231 b < tmp1 > tmp2
./replace.pl 44802 z < tmp2
echo -e "\n\n\n"