my $bar = 3;
my $foo = 'Bar is $bar';
print $foo, "\n";

##</code><code>##

my $bar = 3;
my $foo = "Bar is $bar";
print $foo, "\n";

##</code><code>##

For literal strings, quote with Single.
(Isn't this a silly jingle?)
Interpolation uses Double.
Forget this rule, you'll be in trouble.