I started to walk through the logic, and explain why it's not working as you would like, but there were more than a couple smoking guns, and finally I decided to show you one way I might do it, if I were to use File::Find for this.

I will mention however, that your use of grep is incorrect (you should be passing a useful list to it). You would also avoid some confusion by unpacking your function's args list at the top, as $_[0] within your anonymous sub is not referring to the parent sub's arg list. Also, unless you test whether the thing that File::Find is looking at is a file, you will see directories too. Furthermore, "*.exe", used as a regexp pattern isn't doing what you want.

The following has been tested on Linux, with an appropriate change to the target starting path. ;)

use File::Find;


my @files_found = find_files(
  'C:/Program Files (x86)/Nimsoft/bin',
  [ qr/\.exe$/, qr/\.txt$/ ]
);

print "$_\n" for @files_found;


sub find_files {
  my( $start, $patterns_aref ) = @_;
  my @found;
  
  find(
    sub {
      if( -e && -f ) {
        for my $pattern ( @$patterns_aref ) {
          if( m/$pattern/ ) {
            push @found, $File::Find::name;
            last;  # Matched, so we don't need to test other rules.
          }
        }
      }
    },
    $start
  );

  return @found;
}
[download]

It should work on Windows as well.

Update: I mentioned at the outset that this is one way I might do it if I were to use File::Find. I said that because I would probably prefer using the cleaner feeling File::Find::Rule instead; although I haven't worked through the solution, I suspect it would turn out easier to read.