Make open(... "<&", $fileno) respect magic

A magical variable is never SvPOK, but only SvPOKp.  The code for
checking whether a duplicatee is a numeric file descriptor was only
checking SvPOK.  So a regular variable containing a fileno-as-a-string
would work, such as the $a below, as would a stringified magical vari-
able ("$1"), but not $1 itself.

$ echo foo | perl -le '$a = "0"; open a, "<&", $a; warn <a>'
foo
$ echo foo | perl -le '"0" =~ /(.)/; open a, "<&", $1; warn <a>'
Can't use an undefined value as filehandle reference at -e line 1.
$ echo foo | perl -le '"0" =~ /(.)/; open a, "<&", "$1"; warn <a>'
foo

SvPOK variables are also SvPOKp, so checking only the latter suffices.

1 files changed, 5 insertions, 1 deletions

diff --git a/t/io/open.t b/t/io/open.t
index 696ba98acd..e06fc8e394 100644
--- a/t/io/open.t
+++ b/t/io/open.t
@@ -10,7 +10,7 @@ $|  = 1;
 use warnings;
 use Config;
 
-plan tests => 120;
+plan tests => 121;
 
 my $Perl = which_perl();
 
@@ -233,6 +233,10 @@ like( $@, qr/Bad filehandle:\s+$afile/,          '       right error' );
 
     # used to try to open a file [perl #17830]
     ok( open(my $stdin,  "<&", fileno STDIN),   'dup fileno(STDIN) into lexical fh') or _diag $!;
+
+    fileno(STDIN) =~ /(.)/;
+    ok open($stdin, "<&", $1), 'open ... "<&", $magical_fileno',
+	||  _diag $!;
 }
 
 SKIP: {