Need help with a "Binding Variable" excercise

[jason corbett]

I am trying to learn about binding variables. i am writing a simple script that with 
select a row of records and print them to screen. I am getting the error
Can't call method "bind_param" on an undefined value at porqout.pl line 29.
 
Here is the code below.
 
#!/usr/bin/perl -w
use strict; 
use DBI;
 
my $ctn_number='67846270';

 $dbh->prepare("select * from ctn_inv where ctn = ?");
 $sth->bind_param( 1,$ctn_number);
 
 $sth->execute();
 
$dbh->prepare("select * from ctn_inv where ctn = ?");
 $sth->bind_param( 1,$ctn_number);
 
 $sth->execute();
 
 while( my @record= $sth->fetchrow_array())
 
my $record_list = join ',', @record;
 print "\n$record_list";  
 
my $sth->finish;
 $dbh->disconnect;
 exit;