Aristotle's idea works fine for me.

use strict;
my @input = split /\n/,
'
-IE:\david\nbssbts\btsc\bld -IE:\david\nbsscis\dcsapp\bld \
--cxx_include_directory C:\Green\scxx -IC:\Green\scxx \
-IC:\Green\68000\include -IC:\Green\ansi \
';

foreach (@input) {
    print "$1\n" while /\B(-I\S+)/g;
}

___OUTPUT___

-IE:\david\nbssbts\btsc\bld
-IE:\david\nbsscis\dcsapp\bld
-IC:\Green\scxx
-IC:\Green\68000\include
-IC:\Green\ansi
[download]

Of course this will not work if there are embedded spaces in the path names.

Update --
To handle embedded spaces use a zero-width lookahead.

use strict;
$_ = 
'
-IE:\david\nbssbts\btsc\bld -IE:\david\nbsscis\dcsapp\bld \
--cxx_include_directory C:\Green\scxx -IC:\Green\scxx \
-IC:\Green\68000\include -IC:\Green\ansi \
-ID:\Path with spaces\include \
';

s/\\\n//g;

print "$1\n" while /(-I.+?(?= -I| --c|$))/g;


__OUTPUT__

-IE:\david\nbssbts\btsc\bld
-IE:\david\nbsscis\dcsapp\bld
-IC:\Green\scxx
-IC:\Green\68000\include
-IC:\Green\ansi
-ID:\Path with spaces\include
[download]

--

flounder