#!c:\usr\bin\perl.exe
use warnings;
use strict;
use 5.006;  # this is a lie; really, require 5.8.0 for 3-argument open
+() to dup STDIN

open my $infile, '<',$0 or die "cannot open $0 for read as \$infile: $
+!";
open INFILE, '<', $0    or die "cannot open $0 for read as INFILE: $!"
+;
open OLDIN, "<&STDIN"   or die "cannot dup STDIN to OLDIN: $!";
close STDIN;

open STDIN, '<&', $infile   or warn "cannot set STDIN = \$infile using
+ 3-argument open(): $!";      # 3-argument dup is not valid perl 5.6.
+0-5.6.2
print "READ FROM STDIN=<&\$infile:\n" . <STDIN> . <STDIN> . $/;
close STDIN;
close $infile;

open STDIN, '<&INFILE'      or warn "cannot set STDIN = <&INFILE using
+ 2-argument open(): $!";      # 2-argument dup should be valid perl 5
+.6.0
print "READ FROM STDIN=<&\$infile:\n" . <STDIN> . <STDIN> . $/;
close STDIN;
close INFILE;

open STDIN, "<&OLDIN"       or warn "cannot reset STDIN = <&OLDIN usin
+g 2-argument open(): $!";
close OLDIN;

print "Input Text to ensure we switched back to original STDIN: ";
my $in = <STDIN>;
print "READ FROM STDIN:$/$in$/";
exit;
[download]

(While debugging that, I learned that I'll have to switch from open($fh, '<&STDIN') to open(FILEHANDLE, '<&STDIN') to be able to use the dup feature.)

And it's perlver output:

C:>perlver perlver_bug.pl


   ---------------------------------------------
 | file           | explicit | syntax | external |
 | --------------------------------------------- |
 | perlver_bug.pl | v5.6.0   | v5.6.0 | n/a      |
 | --------------------------------------------- |
 | Minimum explicit version : v5.6.0                        |
 | Minimum syntax version   : v5.6.0                        |
 | Minimum version of perl  : v5.6.0                        |
   ---------------------------------------------


C:>perlver --blame perlver_bug.pl

 ------------------------------------------------------------
 File    : perlver_bug.pl
 Line    : 2
 Char    : 1
 Rule    : _perl_5006_pragmas
 Version : 5.006
 ------------------------------------------------------------
 use warnings;
 ------------------------------------------------------------
[download]

As to why it flagged on 46, but not 42: I'm now reasonably confident it's because 42 was a 2-argument open()/dup, whereas 46 was the invalid 3-argument open()/dup.