use 5.010;
use strict;
use warnings;

sub square(&$) {
  my ($rc, $x) = @_;
  @_ = ($x * $x); goto &$rc;
}

sub square_and_say(&$) {
  my ($rc, $x) = @_;
  my $square_rc = sub {
    my $s = shift;
    say "$x ^ 2 = $s";
    @_ = (); goto &$rc;
  };
  @_ = ($square_rc, $x); goto □
}

square_and_say { } 2;