foreach (sort {&compare} @lines) {
    print "$_";
}

sub compare {
    $a =~ /(\d+)\/(\d+)\/(\d{4}),(.*)/;
    my $c = $3 . $1 . $2; 

    $b =~ /(\d+)\/(\d+)\/(\d{4}),(.*)/;
    my $d = $3 . $1 . $2;

    $c <=> $d;
}