Kludgy, but you could use what you've got and append every even element to the preceding odd element.

my @result;
while( my( $o, $e ) = splice( @tmp, 0, 2 ) ) { push @result, $o . $e }
print join( "\n", @result ), "\n";
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Update: Bleh, ignore me. Look at the other suggestions. Now where'd my caffeine go . . .