use strict;

my ($x,$y,@x,@y,$i);

# case 1 
$x = "YAAY";
$y = "YAAA";
$x =~ s/[$y]+//g;
print "$x";

# case 2
$x = "YAAY";
$y = "YAAA";
@x = split(//,$x);
@y = split(//,$y);
for($i=0;$i<=$#x;$i++){if($x[$i] ne $y[$i]){print "$x[$i]\n"}}