have also tried to undefine @clsses in the loop. However, like above, no success.

for my $name (keys %characters) {
  my $id = $name;
     $id =~ s! !_!g;
     $id =~ s![^-\w:.]!!g; 
  
  my $experience = $characters{$name}{experience};
  my @classes = @{$characters{$name}{classes}};
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You are declaring @classes inside the loop. Therefore, every time through the loop, the @classes variable is destroyed. A new @classes variable is created each time through the loop and it is assigned a new value.

It's not clear to me why you want to 'undefine' @classes. Is it possible you are doing something like this:

use strict;
use warnings;
use 5.010;

my @classes = ('A', 'B');

for (1 .. 3) {
    my @classes = ('C', 'D');
    say "@classes";
}

say "\n@classes";

--output:--
C D
C D
C D

A B
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Anything you do to @classes inside the loop braces only affects the @classes variable that you declared inside the loop braces. Changes to the @classes variable inside the loop braces aren't going to affect the @classes variable declared outside the loop braces.