For reasons I don't understand at the moment, passing \*STDIN and \*STDOUT doesn't work. The following does, though.

my $pid = open3(
   '<&STDIN',
   '>&STDOUT',
   my $fr_chld_err = gensym(),
   'email'
)
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Tested as follows:

File wrapper:

#!/usr/bin/perl

use strict;
use warnings;

use IPC::Open3 qw( open3 );
use Symbol qw( gensym );

my $pid = open3(
   '<&STDIN',
   '>&STDOUT',
   my $fr_chld_err = gensym(),
   'email'
);

while (<$fr_chld_err>) {
   next if /Module 'eAccelerator' already loaded/;
   print STDERR $_;
}

waitpid($pid, 0)
   or die;
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File email:

#!/usr/bin/perl

use strict;
use warnings;

use IO::Handle qw( );

STDOUT->autoflush(1);
STDERR->autoflush(1);

print STDERR ("PHP Warning:  Module 'eAccelerator' already loaded in U
+nknown on line 0\n");

print STDOUT ("[STDOUT]\n");
print STDERR ("[STDERR]\n");

while (<STDIN>) {
   print STDERR (">> $_");
}
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$ echo meow | wrapper
[STDOUT]
[STDERR]
>> meow
[download]