Try

#!/usr/bin/perl
use strict;
use DBI;
use GD::Graph::Data;
use GD::Graph::bars;

my $batchnumber = '20170903nervana';

my $sql = '
  SELECT time, mastuntemp, LineTemp, spargtemp 
  FROM brew_temp_tb 
  WHERE sitename = ? 
    AND batchnumber = ?
  ORDER BY time';

my $dbh = dbh(); # connect
my $sth = $dbh->prepare($sql);
$sth->execute('Brew',$batchnumber);           
           
my $data = GD::Graph::Data->new();
while (my @row = $sth->fetchrow_array){
  $data->add_point(@row);
}

my $chart = GD::Graph::bars->new();
my $gd = $chart->plot($data);
open(IMG, '>','bar.png') or die $!;
binmode IMG;
print IMG $gd->png;

# connect
sub dbh{
  my $database = "test"; 
  my $user     = "user"; 
  my $pw       = "password";
  my $dsn = "dbi:mysql:$database:localhost:3306"; 
  my $dbh = DBI->connect($dsn, $user, $pw, 
     { RaiseError=>1, AutoCommit=>1 } );
  return $dbh;
}
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