prompt> perl
$foo = "cheese";
$bar = "stuff/$foo.txt";
print "Bar is [$bar]\n";
^D
Bar is [stuff/cheese.txt]

##</code><code>##

prompt> perl
$foo = "cheese";
$bar = 'stuff/$foo.txt';
print "Bar is [$bar]\n";
^D
Bar is [stuff/$foo.txt]

##</code><code>##

$bar = "stuff/$foo1234"; # Bad
$bar = "stuff/${foo}1234"; # Good