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sub pin {
    srand;
    my $pin;
...
    }
    return $pin;
}

my @counterexamples;
my %digits = map { $_ => 1 } 0 .. 9;
for ( 1 .. 10000 ) {
...
    or diag +Data::Dumper->Dump( [ \@counterexamples ], ['*bad_pins'] 
+);
ok !%digits, '... and all digits should be used'
    or diag sprintf "Unused digits are '%s'", join ', ', sort keys %di
+gits;

sub pin {
    return sprintf "%04d" => int rand 10000;
}