Is there a way to capture a subroutine's print output to a variable so I can send it to stderr instead?

Question

Suppose we have:

sub test {
        print \"testing\\n\";
}

If there is a case where I want to have it print to stderr instead of stdout, i

Answer 1

Perl's dynamic scoping via local() is not often used, but this strikes me as a good application for it:

test(); # to stdout
{
    open(local *STDOUT, ">&STDERR") or die "dup out to err: $!";
    test(); # to stderr, locally calling it "STDOUT"
}
test(); # to stdout again

The call to test() in the block above -- as well as to anything that test() itself might call -- will have STDOUT dynamically scoped to your duplicate of STDERR. When control leaves the block, even if by die()ing, STDOUT will be restored to whatever it was before the block

Generalized:

sub out2err(&) {
  my $user_block = shift;
  open(local *STDOUT, ">&STDERR") or die $!;
  $user_block->();
}

test();             # to stdout
out2err { test() }; # to stderr
test();             # to stdout