Wrong! Win32::API does no defined check.

Hm, Maybe you're right, but it doesn't seem to be necessary. With your insight about buffering & EOF and a little wrapping, this seems to work quite nicely:

#! perl -slw
use strict;
use Win32API::File qw[ GetOsFHandle ];
use Win32::API::Prototype;

ApiLink(
    'Kernel32', q[
        BOOL PeekNamedPipe(
            HANDLE hNamedPipe,
            LPVOID lpBuffer,
            DWORD nBufferSize,
            LPDWORD lpBytesRead,
            DWORD *lpTotalBytesAvail,
            LPDWORD lpBytesLeftThisMessage
        )
    ]
) or die $^E;

sub readlineMaybe {
    my $fh = shift;
    my $osfh = GetOsFHandle( $fh ) or die $^E;
    my( $bufsize, $buffer, $cAvail, $read ) = ( 1024, chr(0)x1024, 0, 
+0 );
    PeekNamedPipe( $osfh, $buffer, $bufsize, $read, $cAvail, 0 ) 
        or $^E == 109 or die $^E;
    return if $^E == 109;
    my $eolPos = 1+index $buffer, $/;
    return '' unless $eolPos;
    sysread( $fh, $buffer, $eolPos ) or die $!;
    return $buffer;
}

my $cmd = 
  'perl -le"$|++;print localtime().q[: some text] and sleep 1 for 1..1
+0" |';
my $pid = open my $pipe, $cmd or die $!;

while( defined( my $line = readlineMaybe( $pipe ) ) ) {
    Win32::Sleep( 100 ) and next unless $line;
    chomp $line;
    chop $line; ## Annoying!
    print "Got: '$line'";
}

__END__
c:\test>buk-pipe.pl
Got: 'Wed Oct  1 02:01:21 2008: some text'
Got: 'Wed Oct  1 02:01:22 2008: some text'
Got: 'Wed Oct  1 02:01:23 2008: some text'
Got: 'Wed Oct  1 02:01:24 2008: some text'
Got: 'Wed Oct  1 02:01:25 2008: some text'
Got: 'Wed Oct  1 02:01:26 2008: some text'
Got: 'Wed Oct  1 02:01:27 2008: some text'
Got: 'Wed Oct  1 02:01:28 2008: some text'
Got: 'Wed Oct  1 02:01:29 2008: some text'
Got: 'Wed Oct  1 02:01:30 2008: some text'
[download]

The only annoying thing is the need for that chop in addition to chomp.