#!/usr/bin/perl --

use strict;
use warnings;

my $str  = "foo";
my $copy = $str;

print "$copy $str\n";

nocopy($str);
yescopy($str);

print "$copy $str\n";    # now $str is foobar

sub nocopy { $_[0] .= 'bar'; '...' }
sub yescopy { my ($copy) = $_[0]; $copy = shift; $copy .= 'bar' }

__END__
foo foo
foo foobar