///My code
use FileCache maxOpen => 1000;
////////////
# config:
my $field = 0;
my $sep = ",";

////MY code
cacheout $mode, $path;
$fh = cacheout $mode, $path;
/////////


$, = $sep;
$\ = $/;


my %file; # { num, name, $fh }

my $fnum = 1;


while (<>)
{
    chomp;
    my @c = split /$sep/o;

    my( $key, $num ) = defined $c[$field]
        ? ( $c[$field], $fnum++ )
        : ( '(column not present)', 0 );
    
        
    
    unless ( $file{$key})
    {
        $nameF = $c[$field];
        $nameF =~ s/"//g; 
    
        $file{$key}{num} = $num;
        $file{$key}{name} = "out/".$nameF.$ARGV[0];
        if(($file{$key}{num}) >1){
        -f $file{$key}{name} and die
            "Sorry, '$file{$key}{name}' exists; won't clobber."; 
        open $file{$key}{fh}, ">", $file{$key}{name} or die
            "Error opening '$file{$key}{name}' for write - $!";
    }}

    print {$file{$key}{fh}} @c;
}
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I read the FileHandleFileCache documentation differently than you do. I think that you're basically supposed to replace your calls to open by calls to cacheout, that is, instead of open $file{$key}{fh}, ..., use:

$file{$key}{fh} = cacheout $file{$key}{name}
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But I haven't tested that. $path is the (path and) name of the output file, and $mode is the file mode (which is irrelevant to your needs).

Update: kyle spotted a link to the wrong documentation.

I have tried

use FileCache maxOpen => 10000;
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..

open $file{$key}{fh}, ">", cacheout $file{$key}{name} or die
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But I get the error
Too many open files at /usr/lib/perl5/5.10/ .... at line 408948
I have tried changing the value of maxOpen but this does nothing

I'm not sure why you interpret what I wrote:

instead of open $file{$key}{fh}, ..., use:

$file{$key}{fh} = cacheout $file{$key}{name}
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as that you should write:

open $file{$key}{fh}, ">", cacheout $file{$key}{name} or die
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Maybe you want to reread my node again. I'm also not sure how to phrase it differently so you get what I mean short of writing the program for you which I won't do.

use FileCache maxOpen => 10000;
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open $file{$key}{fh}, ">", cacheout $file{$key}{name} or die
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