use List::Util qw( shuffle );

my @passwd;
push @passwd, $letters[ rand(@letters) ] for 1..$num_letters;
push @passwd, $digits[  rand(@digits ) ] for 1..$num_digits;
push @passwd, $symbols[ rand(@symbols) ] for 1..$num_symbols;
push @passwd, $chars[   rand(@chars)   ] for 1..($length-$num_letters-$num_digits-$num_symbols);

my $passwd = join '', shuffle @passwd;