A non-regex version. It grabs every second pair of space delimited strings in the line.

my $data = '1 2 3 4 5 6 7 8 9 1 0'
print join ' ', grep $_,(split' ', $data)[ map{ my $n = 4 * $_; ( $n, 
+$n + 1 ) } 0..5 ];
1 2 5 6 9 1
[download]

Adjust 0 .. 5 to suit the maximum number of items/4