1.15.1. Problem

1.15.3. Discussion

This code shows both techniques. The first line shows concatenation; the second shows the expansion trick:

$phrase = "I have " . ($n + 1) . " guanacos.";
$phrase = "I have ${\($n + 1)} guanacos.";

The first technique builds the final string by concatenating smaller strings, avoiding interpolation but achieving the same end. Because print effectively concatenates its entire argument list, if we were going to print $phrase, we could have just said:

print "I have ",  $n + 1, " guanacos.\n";

When you absolutely must have interpolation, you need the punctuation-riddled interpolation from the Solution. Only @, $, and \ are special within double quotes and most backquotes. (As with m// and s///, the qx( ) synonym is not subject to double-quote expansion if its delimiter is single quotes! $home = qx'echo home is $HOME'; would get the shell $HOME variable, not one in Perl.) So, the only way to force arbitrary expressions to expand is by expanding a ${ } or @{ } whose block contains a reference.

In the example:

$phrase = "I have ${\( count_em( ) )} guanacos.";

the function call within the parentheses is not in scalar context; it is still in list context. The following overrules that:

$phrase = "I have ${\( scalar count_em( ) )} guanacos.";

You can do more than simply assign to a variable after interpolation. It's a general mechanism that can be used in any double-quoted string. For instance, this example builds a string with an interpolated expression and passes the result to a function:

some_func("What you want is @{[ split /:/, $rec ]} items");

You can interpolate into a here document, as by:

die "Couldn't send mail" unless send_mail(<<"EOTEXT", $target);
To: $naughty
From: Your Bank
Cc: @{ get_manager_list($naughty) }
Date: @{[ do { my $now = `date`; chomp $now; $now } ]} (today)

Dear $naughty,

Today, you bounced check number @{[ 500 + int rand(100) ]} to us.
Your account is now closed.

Sincerely,
the management
EOTEXT

Although these techniques work, simply breaking your work up into several steps or storing everything in temporary variables is almost always clearer to the reader.

The Interpolation module from CPAN provides a more syntactically palatable covering. For example, to make elements of the hash %E evaluate and return its subscript:

use Interpolation E     => 'eval';
print "You bounced check number $E{500 + int rand(100)}\n";

Or to make a hash named %money call a suitably defined function of your choice:

use Interpolation money => \&currency_commify;

print "That will be $money{ 4 * $payment }, right now.\n";

expect to get something like:

That will be $3,232.421.04, right now.

1.15.4. See Also