@res = qw/index.cgi/; # could be more than one file...
@ext = qw/css tmp class edf/;
$regex = '';    # note: "$", not "@"

##</code><code>##

perl -e '$_ = "hi"; print "ok\n" if ( /|x/ );

##</code><code>##

$string = "initial value";
for ( @addons ) {
    $string .= " $_";   # or whatever
}

##</code><code>##

my $ext_regex = '\.(' . join('|', qw/css tmp class edf/) . ')$';

for ( @filenames ) {
   print "this one matches: $_\n" if ( /$ext_regex/i );
}