The prototype of readpipe() is now ($). Previously, it was broken:
$ perl5.8.8 -wle 'print prototype"CORE::readpipe"'
Can't find an opnumber for "readpipe" at -e line 1.
$ bleadperl -wle 'print prototype"CORE::readpipe"'
$

##</code><code>##

actarus:~/test [13:02:32]$ ls
foo.pl
actarus:~/test [13:02:35]$ cat foo.pl
#!/usr/bin/perl

use strict;
use warnings;

my %file;

BEGIN {
    no strict 'refs';
    *{'CORE::GLOBAL::stat'} = sub {
        warn "trying mocked stat()\n";
        (my $f)=@_;
        CORE::stat( @_ && !ref($f) &&
                    exists $file{$f} ? $0 : @_ );
    };
}

use Test::More qw(no_plan);

$file{foo} = 1;
ok( (scalar stat $_) => "file $_ exists") for qw/foo bar/;
ok( (-e $_) => "file $_ exists") for qw/foo bar/;

__END__
actarus:~/test [13:02:38]$ ./foo.pl
trying mocked stat()
ok 1 - file foo exists
trying mocked stat()
not ok 2 - file bar exists
#   Failed test 'file bar exists'
#   at ./foo.pl line 21.
not ok 3 - file foo exists
#   Failed test 'file foo exists'
#   at ./foo.pl line 22.
not ok 4 - file bar exists
#   Failed test 'file bar exists'
#   at ./foo.pl line 22.
1..4
# Looks like you failed 3 tests of 4.