foreach isn't what you want here. Assuming you don't want to examine what you've just inserted: 
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
}

[download]
Update: removed parens in $i += ( @in ); though it seems to work.

--Bob Niederman, http://bob-n.com

In reply to Re: Inserting into an array while stepping through it by bobn
in thread Inserting into an array while stepping through it by Anonymous Monk

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