my $check_inv_sql = "SELECT MAX(cdt) AS md FROM s_i ORDER BY cdt LIMIT 1";

$sth=$dbh->prepare($s_query) or die $dbh->errstr;

$sth->execute;

my $mt;

if (my $hash_ref = $sth->fetchrow_hashref)
{
 $mt = $hash_ref->{'md'};
print "$mt\n";#this returns below error
}

##</code><code>##

Use of uninitialized value $mt in concatenation (.) or string at Sb.pl line 56.