You said you were interested in other, simpler, solutions, so here is mine: simple use of built-ins, but no real attempt at "golfing" it.

—John

while (<DATA>) {
   chomp;
   s/(.)(.)/$2$1/g;
   print "$_\n";
   }

__DATA__
This is a test.
This is the second line.
12ab34cd56ef78gh90ij
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