sub input    {                    ...; return $value;  }
sub process1 { my ($input ) = @_; ...; return $result; }
sub process2 { my ($input ) = @_; ...; return $result; }
sub output   { my ($result) = @_; ...;                 }

output(
   process1(
      process2(
         input()
      )
   )
);

##</code><code>##

sub input    { my (        $c) = @_; ...; &$c($value ); }
sub process1 { my ($input, $c) = @_; ...; &$c($result); }
sub process2 { my ($input, $c) = @_; ...; &$c($result); }
sub output   { my ($result   ) = @_; ...;               }

input(
   sub { process1(@_, 
      sub { process2(@_,
         \&output
      ) }
   ) }
);

##</code><code>##

sub input    { my (        $c) = @_; ...; @_ = ($value ); goto(&$c); }
sub process1 { my ($input, $c) = @_; ...; @_ = ($result); goto(&$c); }
sub process2 { my ($input, $c) = @_; ...; @_ = ($result); goto(&$c); }
sub output   { my ($result   ) = @_; ...;                            }

input(
   sub { process1(@_, 
      sub { process2(@_,
         \&output
      ) }
   ) }
);

##</code><code>##

goto line1;

input:    { my (        $c) = @_; ...; @_ = ($value ); goto($c); }
process1: { my ($input, $c) = @_; ...; @_ = ($result); goto($c); }
process2: { my ($input, $c) = @_; ...; @_ = ($result); goto($c); }
output:   { my ($result   ) = @_; ...;                           }

line1: { push(@_, 'line2'); goto input;    }
line2: { push(@_, 'line3'); goto process1; }
line3: { push(@_, 'line4'); goto process2; }
line4: {                    goto output;   }