my %OFH;
my $OFH;
while (<$IFH>) {
    my @fields = split /\t/,$_;
    $OFH = $OFH{$fields[$key_column]};
    if (! defined $OFH) {
        # We don't have this value yet, so open another file
        open $OFH, '>', 'key_value.' . $fields[$key_column];
        $OFH{$fields[$key_column]} = $OFH;
    }
    print $OFH join("\t",@fields);
}