If I had the push, I would put coroutines or at least iterators into the Perl 5 core, like Coro provides them already (in a less portable, more restricted way). With at least iterators, you can easily avoid the "inversion of control" problem that arises as soon as you have or use a framework that either provides callbacks or provides call-in hooks, and you don't need to maintain state yourself (see the push/pull examples below).

Painless iterators:

sub i_produce {
  my ($start, $count) = @_;
  for my $i ($start..$start+$count) {
    yield $i;
  };
};

sub i_two_columns {
  while (defined (my $i = produce)) {
    print $i;
    if (defined (my $j = produce)) {
      print "\t$j";
    };
    print "\n";
  };
};
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Perl way (push):

sub p_produce {
  my ($consumer, $start, $count) = @_;

  for my $i ($start..$start+$count) {
    $consumer->($i);
  };
};

{
  my $odd;

sub p_two_columns {
  # aieeee - need to maintain state
  $odd = ($odd + 1)%2;
};
};
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Perl way (pull):

{
sub p_produce {
  my ($consumer, $_start, $_count) = @_;
  # aieee - need to maintain state:
  my $pos = $start;
  return sub {
    $pos < $start+$count ? $pos++ : undef
  };
};
};

sub i_two_columns {
  my ($produce) = p_produce(2,10);
  while (defined (my $i = $produce->())) {
    print $i;
    if (defined (my $j = $produce->())) {
      print "\t$j";
    };
    print "\n";
  };
};
[download]