# print 
for (my $i =1; $i <= $#arr; $i++) {
#                  ^ missing
   print join(',', @{$arr[$i]})."\n";
#                   ^        ^ missing   
}

##</code><code>##

Output     Required
a,2        a,2
b,f        b,f  
c,4        c,4
s          ,s
w          ,w