Assuming your module takes over STDERR, and then relies on the behavior of croak (or, really, die) to write to STDERR, I don't think you'll get your tests to work with eval, which supresses the write to STDERR. You may be better off using a signal handler:

local $SIG{__DIE__} = sub { 
  local $|++; # autoflush
  print STDERR @_; 
};

unlink 'test.errors';

redirect_error( output_file => 'test.errors' ); 

# call this directly, not in an eval, as you're trapping the croak
lcroak "this is a sample error file";

ok( -T 'test.errors',
    "lcroak opens the error file specified by redirect_error()." );

my $text = `cat test.errors`;
like( $text , qr/this is a sample error file/,
    "lcroak writes to the error file specified by redirect_error()." )
+;

unlink 'test.errors';
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Picking a nit -- cat test.errors isn't portable. You may want to use File::Slurp or the equivalent and keep it in Perl.

Also, if you want to do this test by running a separate process, I'd suggest trying IPC::Run3 instead of using a system call. It will nicely redirect STDOUT and STDERR into scalar variables for you:

run3 \@cmd, undef, \$out, \$err;
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