$ perl -MO=Deparse -F: -lane 'push @x, $F[0] if /^[A-Z]/}{print for sort @x'

BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = readline ARGV)) {
    chomp $_;
    our @F = split(/:/, $_, 0);
    push @x, $F[0] if /^[A-Z]/;
}
{
    print $_ foreach (sort @x);
}