#!/usr/bin/perl

use warnings;
use strict;
use DBI;

my $dbh;

#A lexical scope so that the db... variables go out of scope quickly.
{
        my $dbname = "database";
        my $dbuser = "user";
        my $dbpass = "password";
        my $dbhost = "localhost";
        my $dbport = 3306;
        my $dsn    = "DBI:mysql:database=$dbname;host=$dbhost;port=$dbport";
        $dbh = DBI->connect($dsn, $dbuser, $dbpass, {  RaiseError => 1 })
                or die "Couldn't connect to the database: $!\n";
}


#Use placeholders here. Your DBD driver will then take care of
#appropriate quoting.
my $sth = $dbh->prepare("SELECT ID, Data FROM table_name WHERE Data=?");
$sth->execute("value");


#You're executing the update query lots of times.
#Prepare it once, then execute it with different values.

my $update_sth = $dbh->prepare("UPDATE new_table SET newData=? WHERE newID=?");

#No point doing the looping over your DataHash twice, you've got the data
#once, why not use it here?
while(my ($id, $data) = $sth->fetchrow) {
        $update_sth->execute($data, $id);
}

#I never bother with these if it's at the end of a script personally.
#They'll happen automatically when the variables get DESTROY'ed
$sth->finish;
$update_sth->finish;
$dbh->disconnect;