use Tie::File;

my $id_from_form=3;
my $move="up";
my $flat_file = 'path/to/flat.file';

tie my @array, 'Tie::File',$flat_file or die $!;
my $index = grep{$_->[0] =~ /^$id_from_form\*\*/}@array;
if($move eq 'up'){
   my $elem = $array[$index];
   $array[$index] = $array[$index-1];
   $array[$index-1] = $elem;
}
else{
   my $elem = $array[$index];
   $array[$index] = $array[$index+1];
   $array[$index-1] = $elem;
}
untie @array;