use strict;
use warnings;

for my $number (1233456, 10, 1, 555789041) {
    my %digits;
    my @failDigits = grep {++$digits{$_} == 2} split '', $number;
    
    next if ! @failDigits;
    
    print "Number $number failed for:\n";
    print "   $_: $digits{$_} places\n" for @failDigits;
}

##</code><code>##

Number 1233456 failed for:
   3: 2 places
Number 555789041 failed for:
   5: 3 places