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...)

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).

Hey!!! Its Working thanx!!! monks... :)

Update
My mistake, obviously the tr operator is not a good choice. The substr function or the s operator as suggested below both are fine.

How would you do this with tr///?

Good point, obviously I wouldn't;)