my @arr = qw(abc aaa ccc bbb ddd);
for (my $i=0; $i<=$#arr; $i++) {
  my @tmp = replace($arr[$i]);
  splice @arr, $i, 1, @tmp;
  $i += $#tmp;
}

print "@arr\n";

sub replace {
  local $_ = shift;
  return
    /^aaa$/ ? qw(rep1 rep2 aaa)
  : /^bbb$/ ? ()
  : $_;
}