printStatement('foo');

sub printStatement { 
	# @_ is ('foo')
	my $var = shift;
	# $var is now what was $_[0], the first item in @_, ie 'foo' 
	print $var;
}

# or just

sub printStatement { print shift }

##</code><code>##

$HelloWorld::var = 'foo';
printStatement();

##</code><code>##

my $important_message = 'foo';
printStatement(\$important_message);

sub printStatement { 
	my $var = shift;
	print $$var;
}