my $lastdate;
foreach my $date
  # n.b. $a and $b interpreted specially in a sort block
  ( sort { sFormat( $a ) <=> sFormat( $b ) }
    keys %datecount
  ) {
    print "\n", $date, ": ", $datecount{$date};
    $lastdate = $date; # overwritten in order until last
}

sub sFormat { # convert dd/mm/yyyy into yyyymmdd
    my $old = shift;
    $old =~ /^(\d{2})\/(\d{2})\/(\d{4})$/
        or die "Programming error: Invalid format $old\n";
    return $3 . $2 . $1;
}