I'm assuming that you want to convert each VCF record into the genetic variant observed, so "0/0" is considered "C" and "1/1" is "A" and "0/1" is "CA".

#!/usr/bin/perl

use strict;
use warnings;

while (<>) {
  if (/^#/) {
    print;
    next;
  }
  chomp;
  my @a = split (/\t/, $_);
  my $ref = $a[3];
  my $alt = $a[4];
  for (my $i=9; $i <= $#a; $i++) {
    if ($a[$i] eq "0/0") {
      $a[$i] = $ref;
    } elsif ($a[$i] eq "1/1") {
      $a[$i] = $alt;
    } elsif ($a[$i] eq "0/1") {
      $a[$i] = "$ref$alt";
    } else {
      $a[$i] = "-";
    }
  }
  print join "\t", @a;
  print "\n";
}

this produces:

$ perl convertVariant.pl < file.vcf
#CHROM  POS     ID      REF     ALT     QUAL    FILTER  INFO    FORMAT  R4157   R4158   R4163
chr7    30902031        .       C       A       .       .       PR      GT      C       C       C