Hm, think about it this way:

open my $FILE, '<', $name_of_file_with_functions_in_it or die ("Crap: 
+$!");

while (<$FILE>) {
   s/\bC\b/$z/g; # replace the *word* "C" with the contents of $z
   print $_;
}

close $FILE;
[download]

This prints the output to STDOUT

If you want to replace 'C' with the literal string '$z', then you just have to escape the $ in the above (\$z).