You might want to consider a simpler more SQL solution

#!perl
use strict;
use DBI;
use Data::Dumper;

my $dbh = create_db();

my $sql = 'SELECT P.common_name,S.plant_size,S.unit,G.abbr
           FROM plants as P
           LEFT JOIN sizes as S
             ON S.plantid = P.id
           LEFT JOIN growers as G
             ON  S.plantid    = G.plantid
             AND S.plant_size = G.plant_size';
my $sth = $dbh->prepare($sql);
$sth->execute();
my %hash=();
while (my @f = $sth->fetchrow_array){
  $hash{$f[0]}{$f[1].$f[2]}{$f[3]} = 1
}
print Dumper(\%hash);
[download]

I appreciate the SQL approach. It's so much easier the building stuff in raw Perl. My SQL chops aren't what yours are, but I'm going to dissect this to see how it works. Thanks.

—Brad
"The important work of moving the world forward does not wait to be done by perfect men." George Eliot