$ perl -e '$foo = qx{echo `ulimit -n`}; print $foo;'
1024
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On some OSen the ulimit command is solely a shell builtin, not an external program. When you try and run it via backticks Perl is looking at your command and thinking it can cheat and run the command itself rather than invoking a shell (since it's a simple command with spaces as the only shell metacharacter). If your OS is one of those without a separate ulimit, what's really happening is that Perl tries to exec the non-existent program and fails (giving you back an undef, but you don't check for that).

The work around is to explicitly get the shell involved, either by putting some shell metacharacters in your command (qx/ulimit -n 3>&-/) or by invoking a shell explicitly (qx/bash -c "ulimit -n"/).

Update: Tweaked sample metacharacters in first example.

Your first example doesn't work for me:

perl -e 'print qx/ulimit -n 2>&1/;'

prints nothing.

Update: Well, you update faster than I complain. Your new first example is dandy! ;-)

The docs for `STRING` say:

A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent.

bash-3.00$ perl -w -e '$foo = `ulimit -n`; print $foo;'
Can't exec "ulimit": No such file or directory at -e line 1.
Use of uninitialized value in print at -e line 1.
bash-3.00$
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I'm not sure if there are other shell equivalents of ulimit or if you can fool perl into using bash somehow.

Hope this helps.

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";
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#!/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;
  }
}
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