open(DAT, 'file.dat') or die "Can't open file.dat: $!\n";

my %data;
while (<DAT>) {
    chomp;
    my ($key, @coords) = (split(/,/))[2, 0, 1];

    push @{$data{$key}}, \@coords;
}

foreach (keys %data) {
    print "$_\n";

    local $" = ', ';
    foreach my $coord (@{$data{$_}}) {
	print "@$coord\n";
    }
    print "end\n";
}
print "end\n";