for (my $i=0; $i < 5; $i++){
    opendir(DIR,$i) or die();
    foreach my $file (readdir(DIR)) {
        # if the filename ends in "test.txt"...
        # adjust the regex as needed
        if ($file =~ /test\.txt$/) {
            print "$i, -${file}-\n";
        }
    }
    closedir(DIR);
}

##</code><code>##

use Benchmark;
timethese(10000, {
    'readdir' => sub {
        my @blah;
        for (my $i=0; $i < 5; $i++){
            opendir(DIR,$i) or die();
            foreach my $file (readdir(DIR)) {
                if ($file =~ /test.txt$/) {
                    push(@blah,$file); 
                }
            }
            closedir(DIR);
        }
    },
    'glob' => sub { 
         my @blah;
         for (my $i=0; $i < 5; $i++){
             for (<$i/*test.txt>) {
                 push(@blah,$_);
             }
         }
 
    }
}
);

Results on my perl 5.6.x box:

Benchmark: timing 10000 iterations of glob, readdir...
      glob: 12 wallclock secs ( 
         3.24 usr +  2.22 sys =  5.46 CPU) 
             @ 1831.50/s (n=10000)
   readdir:  3 wallclock secs ( 
        1.54 usr +  1.69 sys =  3.23 CPU)
             @ 3095.98/s (n=10000)

on a 5.005_003 box (1000 iterations, because it's so slow):

Benchmark: timing 1000 iterations of glob, readdir...
      glob: 216 wallclock secs 
            ( 1.83 usr  9.21 sys + 
            63.22 cusr 107.38 csys =  0.00 CPU)
   readdir:  1 wallclock secs ( 
             0.43 usr +  0.37 sys =  0.80 CPU)