This question gets asked a lot, and the answer is almost always "don't do that".

It's better to use a hash-of-hashes instead:

# set up the base hash
my %hashes = ();

# in the loop:
my $date = $4.$3.$2;
if (my $hashref = $hashes{$date}) {
    print "<TD class=table_font>$date - Hash exists</TD>\n";
    # play with hash
} else {
    my $hashref = {}; # create a new empty hash
    $hashes{$date} = $hashref; store the hashref in the main hash
    print "<TD class=table_font>$date - Hash doesn't exist</TD>\n";
    # play with hash
}
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See also perlreftut and perldata.