The code I used (if anyone cares) is:

genfiles.pl

#!/usr/bin/perl -w
use strict;
use warnings;

for (1 ... 50000) {
    my $t = int rand 10000;
    my $u = join('',('A'..'Z')[rand 26, rand 26, rand 26]);
    open my $OUF, '>', $t.'_'.$u or die;
    close $OUF;
}
[download]

groupfiles.pl

#!/usr/bin/perl -w
use strict;
use warnings;

my %filegroups;

opendir(my $DIRH, '.') || die "Can't open dir: $!\n";
my $cnt=0;
while (my $filename = readdir($DIRH)) {
        next unless $filename =~ /^(\d+)/;
        push @{$filegroups{$1}}, $filename;
        ++$cnt;
}

my $cnt2=0;
for my $grp (keys %filegroups) {
        print "$grp: ", join(", ", @{$filegroups{$grp}}), "\n";
        last if $cnt2++>5;
}

print "Files: $cnt, Groups: ", scalar(keys %filegroups), "\n";
[download]

...roboticus