This is because methods aren't interpolated within strings. There are various tricks to get around this but they're usually used only within the context of golfing, obfu or job security situations e.g

my $f = sub { return 'a sub' };
print "\$f is ${\$f->()}\n";

__output__

$f is a sub
[download]

As you can see this is not very nice, so I'd go with these friendler solutions

my $f = sub { return 'a sub' };
# list
print "\$f is ", $f->(), "\n";
# concat
print "\$f is ". $f->(). "\n";
# save return
my $ret = $f->();
print "\$f is $ret\n";
[download]

HTH

_________
broquaint

update: removed initial example as it could be confusing