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";

##</code><code>##

#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";