#!/usr/bin/perl
use warnings;
use strict;

my @maybe = qw( abc bca cab cba bac acb aab abbc acc );

print "$_ ", ok($_) ? 'good' : 'no good', "\n" for @maybe;

sub ok{ 
    return y/a// > 1 ? 0 : y/b// > 1 ? 0 : y/c// > 1 ? 0 : 1;
}