# count length of the strings
my %len;
foreach $nr (keys %data) {
    foreach my $att (@attr) {
        $len{$att} = 0;         # <====== here
        update_max(%len, $att, $data{$nr}{$att});
    }
}

##</code><code>##

sub update_max(\%$$) {
    my ($ref_data, $att, $val) = @_;

    if (! $ref_data->{$att} ) {
        $ref_data->{$att} = 0;
    }

    my $siz  = $ref_data->{$att};
    my $siz2 = length($val);

    if ($siz < $siz2) {
        $ref_data->{$att} = $siz2;
    }
}