#!/usr/bin/perl
use strict;
use warnings;

sub prefix_line {
	my $lineno = shift @_;
	my $line = shift @_;
	
	#            Odd line?     Yes    No
	my $prefix = $lineno % 2 ? q{+} : q{o};
	print qq{$prefix };
}

sub print_line {
	my $lineno = shift @_;
	my $line = shift @_;
	
	# Print our actual line
	print qq{$line};
}

my @actions = (
	\&prefix_line,
	\&print_line,
);

my $lineno = 0;
while (my $line = <>) {
	$lineno++;
	foreach my $action (@actions) {
		$action->($lineno, $line);
	}
}