Rather than split, I'd be more inclined to use a simple regex...

my $line   = 'Filesystem           1K-blocks      Used Available Use% 
+Mounted on';
my @fields = $line =~ /(\s*\S+)/g;

print "[", 
  join( "][", @fields ), 
  "]\n";

__END__
[Filesystem][           1K-blocks][      Used][ Available][ Use%][ Mou
+nted][ on]
[download]

    --k.