use Date::Manip;
use POSIX;

@data = ([20070410, 20070415, 20070417], [ 10, 20, 15 ]);
for my $i (0 .. $#{$data[0]}-1) {
   my $lhs = &UnixDate(&ParseDate($data[0][$i]), '%s');
   my $rhs = &UnixDate(&ParseDate($data[0][$i+1]), '%s');
   my @gap = ();

   REDO: {
       if( $lhs < $rhs ) {
           $lhs += 86400;
           push @gap, strftime('%Y%m%d', $lhs);
           redo REDO;
       }
   }

   # I'm not 100% on these splices, be sure to use your brain
   # untested, etc etc
   
   splice @{$data[0]}, $i+1, 0, @gap;
   splice @{$data[1]}, $i+1, 0, map {0} @gap;
}
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UPDATE: Although I believe this would work correctly for the first gap, it definitely wouldn't for the next as the $#{$data[0]} would change dramatically after filling the first gap and the for loop would be unaware of it...