That depends on what you're searching. A database will more or less do it for you via DBI and SQL. Text search in files may use system grep or an index of some kind, or can use pure perl. Detail what you want.

If a regex is needed, you can join the search terms with '|' to make an alternation in the regex. That will find a match each time it finds one term.

Your "standard" treatment of the posted form is not so standard any more. Just saying,

use CGI;
my $q = CGI->new;
[download]

takes care of all that.