I think Joost has given the reason for your undefined values warnings. Let me suggest that you use DBI placeholders also:

     
          
my $sth = $dbh->prepare("
    INSERT INTO webpages_data VALUES(?,?,?,?)
");
for(@links) {
    $sth->execute($a,$_,$text[$v],undef);
    $a++;
    $v++;            
}
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Also, stay away from variables named "$a" or "$b" since those have a special meaning for sort().