Rather than relying on the shell, if you have BSD::Resouce installed, you could:

#!/usr/bin/perl

use strict;
use warnings;

use BSD::Resource;

my $max = getrlimit( RLIMIT_NOFILE );
print "$max\n";
[download]

or if you're feeling really adventurous, with Inline::C

#!/usr/bin/perl

use Inline C;
use strict;

my $max = get_max_files();
print $max, "\n";

__END__
__C__
#include <sys/resource.h>

int get_max_files() {

  struct rlimit rl;
  if (getrlimit( RLIMIT_NOFILE, &rl) == 0) {
    return rl.rlim_cur;
  } else {
    return -1;
  }
}
[download]

(that needs more error checking).