It depends a great deal on what you consider to be 'easy' and probably also it depends what it is that you actually want to achieve. The following code may help:

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;
}
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Prints:

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