I like pc88mxer's solution better than my own, but in the interest in showing different ways of doing the same thing, I'll post it anyway.

use Data::Dumper;

my $in = '0 X X X X X X X'
      . ' X 1 X X X X X X'
      . ' X X 2 X X X X X'
      . ' X X X 3 X X X X'
      . ' X X X X 4 X X X'
      . ' X X X X X 5 X X'
      . ' X X X X X X 6 X'
      . ' X X X X X X X 7';

print Dumper [ string2matrix( $in, 8 ) ];

sub string2matrix {
    my ( $string, $width ) = @_;

    my @out;
    while ( $string ) {
        my @iter = split /\s+/, $string, $width + 1;
        $string = scalar @iter > $width ? pop @iter : '';
        push @out, \@iter;
    }

    return @out;
}
[download]