I want to repalce "C" in these functions by $z

Replace with a literal '$z':

my @funcs = (
    '$A = B * C;',
    '$P = D+E+C;'
);

for my $func (@funcs) {
    $func =~ s/C/\$z/g;
    print "func : $func\n";
}
__END__
func : $A = B * $z;
func : $P = D+E+$z;
[download]

Or, replace with the value of a variable $z:

...
my $z = 99;

for my $func (@funcs) {
    $func =~ s/C/$z/g;
    print "func : $func\n";
}
__END__
func : $A = B * 99;
func : $P = D+E+99;
[download]

(Also note that, because $func is an alias to the original value, the values in @funcs would be modified here. If that's not intended, you'd have to make a copy of the value before doing the substitution...)