What is the preferred way of interpolating a constant in a here doc?

Question

I'm sure there are several ways of getting the value 'bar' to interpolate in the <> below, but what is the cleanest way, and why?

use constant FOO => 'bar';

my $msg = <<EOF;
Foo is currently <whatever goes here to expand FOO>
EOF

Answer 1

There are two kinds of here-docs:

• <<'END', which behaves roughly like a single quoted string (but no escapes), and
• <<"END", also <<END, which behaves like a double quoted string.

To interpolate a value in a double quoted string use a scalar variable:

my $foo = "bar";
my $msg = "Foo is currently $foo\n";

Or use the arrayref interpolation trick

use constant FOO => "bar";
my $msg = "Foo is currently @{[ FOO ]}\n";

You could also define a template language to substitute in the correct value. This may or may not be better depending on your problem domain:

my %vars = (FOO => "bar");
my $template = <<'END';
Foo is currently %FOO%;
END

(my $msg = $template) =~ s{%(\w+)%}{$vars{$1} // die "Unknown variable $1"}eg;