This is a perfect use case for a dbm like DB_File, DBM::Deep or even SDBM_File (that is the worst of the lot, but comes with Perl). Then you can have a hash that is transparently mirrored in a file, and accessing it will be much more efficient than your solution.

That would look something like this (untested):

#!/usr/bin/perl
use strict;
use warnings;
use WWW::Mechanize;
use HTML::TokeParser;
use MIME::Lite;
use Fcntl;
use SDBM_File;
tie my %url_seen, 'SDBM_File', 'urls.dbm', O_RDWR|O_CREAT, 0666);

while (1) {
   my $msg;
   my @search = ("broken hdtv","fix hdtv","does not work hdtv");
   my $connection = WWW::Mechanize->new();

   foreach my $term (@search) {
       $connection->get("http://portland.craigslist.org/");
       $connection->form_number(1);
       $connection->field("query",$term);
       $connection->click();

       my @link = $connection->find_all_links(url_regex => qr/(ele|zip
+)\/[0-9]+/i);

       foreach my $link (@link) {
         my $tempLink=$link->url_abs;
         chomp $tempLink;
         if (not $url_seen{$tempLink}) {
            $url_seen{$tempLink} = 1;
            $msg = MIME::Lite->new(
               To       =>'xxxxxxxxxx@vtext.com',
               Subject  =>'CL Link',
               Type     =>'text/plain; charset="iso-8859-1"',
               Data     =>"There is a new link on craigslist."
            );
            close OUTFILE;
         }
       }
   }
   if ( $msg ) {
     $msg->send;
   }
   sleep(300);
}
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