Update: I didn't change the code, I just reformatted the comments for easier reading. All comments immediately precede the relevant code. I think the suggestions above are better, but if you really want to get this working, I think this might work:

#!/usr/bin/perl -w

use strict;

use File::Find;
my ($string, @files);

#filename will match anything if we haven't specified an extension

my $ext = "."; 
($string, @files) = @ARGV;

#if it begins with a dot or an asterisk dot, it's an extension

if (@files && $files[0] =~ /^(\.|(\*\.))/) { 

    # join all extensions in regex suitable string

    $ext = join ('|', @files); 

    # remove the asterisks and escape the dots

    $ext =~ s/(\*|\.)/($1 eq "*")? "":"\\."/ge;     
    @files = (".");
}
@files = "." unless @files;

find sub {

    # skip if it doesn't end in $ext

       unless ((-x) || ($_ !~ /($ext)$/io)) { 
          if (open (FILE, "$_")) {
             while (my $line = <FILE>) {
                print "$_: $line\n" if $line =~ /$string/io; 
         }
          close FILE;
        } else {
          warn "Can't open $_: $!";
          next;
        }
       }
}, @files;
[download]

It works just like before, but if you use extensions instead of filenames, it will search only extensions. It's an XOR situation: only extensions or filenames, not both.

Extensions may or not be preceded by an asterisk. Your choice. This is an example that I used:

perl grep.pl findxyz .htm *.pl
[download]

I know it's a hack, but it was fun!