Here's the problem (or one of them):

while (<USEDPW>)
    {
        my $pw = @chars[map{rand @chars} (1..17)];
    }
unless ($_ ne my $usedpw) {
        print "Your unique ID is: $pw\n";
    }

my $pw;
open(USEDPW, "> $usedpw") or die "crap on me for writing $!";
flock USEDPW, 2;
print USEDPW "$pw\n";
[download]

You have 2 separate variables named $pw. You used my which is a Good Thing but forgot that my lexically scopes your variables. That means that the my $pw inside the while(<USEDPW>) loop is a completely different variable than the one declared outside the loop. pfaut fixed it in his post but didn't tell you why he did it. You need to declare the $pw variable outside the loop so it stays in scope. For example:

#notice we declare it OUTSIDE the loop
my $pw;
while (<USEDPW>)
    {
        $pw = @chars[map{rand @chars} (1..17)];
    }
unless ($_ ne my $usedpw) {
        print "Your unique ID is: $pw\n";
    }

open(USEDPW, "> $usedpw") or die "crap on me for writing $!";
flock USEDPW, 2;
print USEDPW "$pw\n";
[download]

You should have gotten a "Use of uninitialized value in print" warning when you ran your code.