Good Morning agent_smith, I would address this problem by building a hash of file handles as I read through the data. The hash keys would be the dates, and the hash values would be the file handle (references). I might write it something like this: (warning, untried)

my %file_handles = ();
while (<>) {
    chomp;
    my @line = split /;/;
    if (not defined $file_handles{$line[0]} ){
        open $file_handles{$line[0]}, ">$line[0].data";
    }

    print $file_handles{$line[0]} join "\t", @line;
}
[download]

To close them, just loop through the hash and close each entry.