To clear things up concerning $sth, you'll have to say my $sth; before the eval instead of just adding my before the assignment like castaway suggested you do for $dbh, or else the variable won't exist when it comes to calling $sth->excute.

use CGI;
use CGI::Carp qw(fatalsToBrowser);
use DBI;
use strict;

my $db_name="scent";
my $o = new CGI;
my $tip2 =$o -> param("tip2");

my $DSN="dbi:mysql:$db_name";
my $user="root";
my $pass="";
my $dbh = DBI -> connect($DSN,$user,$pass);  <------


#return;
my $SQL= "select * from par;";

my $sth;   <------
eval {
    $sth = $dbh -> prepare($SQL);
    };
if ($@) {
    $dbh -> disconnect;
    print $@
        }
else {
     $sth -> execute;
     }
[download]

or even:

use CGI;
use CGI::Carp qw(fatalsToBrowser);
use DBI;
use strict;

my $db_name="scent";
my $o = new CGI;
my $tip2 =$o -> param("tip2");

my $DSN="dbi:mysql:$db_name";
my $user="root";
my $pass="";
my $dbh = DBI -> connect($DSN,$user,$pass);  <------


#return;
my $SQL= "select * from par;";

my $sth = eval { $dbh -> prepare($SQL) };   <------
if ($@) {
    $dbh -> disconnect;
    print $@
        }
else {
     $sth -> execute;
     }
[download]