// translate an ASCII file of numbers to acsii characters

#include 
#include 

int main(void){
  int ch,i,letter;
  char num[4];

do {
  for (i=0;i<4;i++){
    num[i] = '\0';
  }
  do{
    ch = getchar();
  }
  while ((ch == ' ') || (ch == '\n') || (ch == EOF));
  i = 0;
  while ((ch != ' ') && (ch != '\n') && (ch != EOF)){
    num[i] = ch;
    ch = getchar();
    i++;
  }
  letter = atoi(num); 
  printf("%c",letter);
  if (ch != EOF)
    printf("%c",ch);
} while (ch != EOF);
}
________________________________________________________________________

Perl Version below

#!/usr/local/bin/perl -w

while(<>){
    s/\n//;
    s/(1[012] [0-9])/chr($1)/ge;
    s/0?([0-9][0-9])/chr($1)/ge;
    print;
}