Or without even Perl . . .

$ irb -r enumerator
irb(main):001:0> "12345678".split(//).enum_slice( 2 ).inject( [] ) { |
+a,x| a << x.join( "" ) }.join( " " ) 
=> "12 34 56 78"
irb(main):002:0> "123456789".split(//).enum_slice( 2 ).inject( [] ) { 
+|a,x| a << x.join( "" ) }.join( " " )
=> "12 34 56 78 9"
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Update: Or maybe a little more concisely:

irb(main):011:0> "12345678".split(//).enum_slice( 2 ).map {|a| a.join(
+"")}.join(" ")
=> "12 34 56 78"
[download]