use strict;
use warnings;
my $a = '12345678901';

# Create a hash which has a count of the number
# of times a character appears in the string.
my %tally;
$tally{$_}++ for split '', $a;

#
# Find hash keys that only have 1 match
#
my @unique = grep {$tally{$_} == 1} keys %tally;
print "Unique keys:\n";
print "$_\n" for sort @unique;