带分隔符的连接字符串

$ ls /tmp/foo/
foo10.txt  foo11.txt  foo15.txt

$ cat ./foo.pl 
use warnings;
use strict;
use Cwd;
my $dir = cwd();
chdir '/tmp/foo';
my @files = glob "foo*.txt";
my $b = "";
for (0..$#files) {
  my ($a) = $files[$_] =~ m/foo(.*)\.txt/;
  $b = $b.",".$a;
}
chdir $dir;
print "$b\n";

输出:

$ perl ./foo.pl 
,10,11,15

我如何避免第一个逗号,就在10之前?另外,请建议是否有比这更好的逻辑。