Since there is More Than One Way To Do It, you could also go for the object-oriented approach using DirHandle:

use DirHandle;

my $dh = new DirHandle;
$dh->open( 'MainDir' );
foreach ( grep /\.cpp$/, $dh->read ) {
   # do stuff
}
$dh->close;
[download]

The DirHandle module is a wrapper around opendir and its relatives, so this is essentially the same as CubicSplines answer, just another way of writing it.

Cheers,
--Moodster