Check which one is closer to your requirements:

$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";
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----------- OUTPUT ----------------------

First interpret
String: abcz123def456
Next:   abda124deg457

Second interpret
String: abcz123def456
Next:   bcda234efg567
[download]