my %handles;

while (<IN>) {
    my $file_out = get_the_way_out($_);
    my @some_data = get_the_data($_);

    if (!exists $handles{$file_out}) {
        open my $handle,'>',$file_out or die $!;
        $handles{$file_out} = $handle;
    }

    # but this is not allowed
    say $handles{$file_out} join "\t",@some_data;
}

close $_ foreach values %handles;