Perl cookbook — interpolating functions within strings

print \($n + 1)

will show you SCALAR(0x24843d0), which denotes a scalar reference.

${\($n + 1)}

will dereference the scalar ref, turning it into a scalar.

The parentheses () are a bit tricky here. They are the list constructor, so one could think that a reference to a list would give an array reference. But that's not the case. In fact, \(1, 2, 3) will give you a list of three scalar references. Likewise, \(1) will be a list of one scalar reference.

This is explained in the Making References part of perlref.

Taking a reference to an enumerated list is not the same as using square brackets--instead it's the same as creating a list of references!

@list = (\$a, \@b, \%c);
@list = \($a, @b, %c);      # same thing!

Using this ref/deref construct inside an interpolation is a trick to get Perl executed and more complex expressions evaluated inside of interpolation. Without it, there would be no way to get $n + 1.

But note that this is not the most readable code. It would be preferable to either break the string into pieces like this

my $phrase = "I have " . $n + 1 . " guanacos."; # or with single quotes ''

or to use sprintf.

my $phrase = sprintf "I have %d guanacos", $n + 1; # or with single quotes ''

Both of these are way more readable, do the same thing, and there is no measurable difference in how fast they are. Clean code should always be your primary objective. Being clever is nice sometimes, but usually you write code for humans first.

You should probably read perlref and perlreftut.