Checking for existence in records?

I'm trying to check if an input record exists in a table, and if it doesn't, add it, or if it does, go on to the next input. The code I wrote is:
   #my $duplangveritem = $dbh->selectrow_array("SELECT baseitemid FROM langversions WHERE baseitemid ='$baseitemid' AND langid='$langid'");
   my $sth = $dbh->prepare("SELECT baseitemid FROM langversions WHERE baseitemid ='$baseitemid' AND langid='$langid'");
   $sth->execute();
   my $duplangveritem = $sth->fetchrow_array;

   if ($duplangveritem) {
      print STDERR "Duplicate item found at line $ln: $_\n";
   } else {
      my $sth = $dbh->prepare("INSERT INTO langversions (baseitemid, langid, title, cost, available) VALUES (?, ?, ?, ?, ?)" ) or die "Can't prepare statement: $DBI::errstr";
      $sth->execute($baseitemid, $langid, $title, $cost, $available)
         or die "Can't execute statement: $DBI::errstr";
   } # else if there was no duplicate item

I tried both the first, commented-out line, or the next three lines, but the line with 'if ($duplangveritem) {' still causes errors like these (the 'if ($duplangveritem)' line is line 72):
Use of uninitialized value in concatenation (.) or string at ./loadInventory.pl line 72. #NOTE: This is unwanted error
Duplicate item found at line 2:                                                                                          #NOTE: This is intended output
Use of uninitialized value in concatenation (.) or string at ./loadInventory.pl line 72.
Duplicate item found at line 15: 

How can I check if the item already exists in the database, and not get an error if it doesn't?

Thanks for all your help.

-Kevin Zembower

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