my @a = qw( 1 3 2 4);
# NOT TESTED MUCH
$i = 0;
while ($i < @a )
{
  if ($a[$i] == 2)
  {
    my @in = mysub(whatever);
    splice @a, $i, 1, @in;
    $i +=  @in ;
  }
  else
  {
     $i++;
  }
}   

print "@a\n";

sub  mysub
{
  # whatever
  return ( qw(9 2 9)); 
  # always return an array, even if only 1 element
}