$str="abcz123def456";

print "\nFirst interpret";
@arr=($str=~m/([a-zA-Z]+|[0-9]+)/g);
$nextstr= join("", map {++$_} @arr);
print "\nString: $str";
print "\nNext:   $nextstr";

print "\n\nSecond interpret";
@arr=split(//,$str);
$nextstr=join("",map{++$_;$_=substr($_,-1,1);} @arr);
print "\nString: $str";
print "\nNext:   $nextstr";