# Main program:

#!/usr/bin/perl

use strict;
use warnings;

sub double {
    my $n = shift;
    return $n * 2;
}

return 1 if caller;  # Don't leave this out.

print join ' ', map {double($_)} @ARGV;
print "\n";

__END__

# Test program:
#!/usr/bin/perl

use strict;
use warnings;

use Test::Simple tests => 3;
    
require 'main.pl';  # Substitute the name of your program.
 
ok(double( 2) ==  4);
ok(double(-3) == -6);
ok(double( 0) ==  0);

__END__