That doesn't work. You're taking an array slice (incorrectly), then assigning it to a scalar variable. You want something like this:

my @chars = ('a'..'z', 'A'..'Z', 0..9, '.', '/');
my $salt = join '', @chars[rand @chars, rand @chars];
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Note the "@" before "chars"--that denotes an array slice. And since we're taking an array slice and want to end up with a scalar, we need to do a join.