In case you're also wondering more generally why it's possible to assign something to a function, the answer is that it is an "lvalue" sub. Another notable example of such a function is substr:

my $foo = "Hello, World!";
substr($foo,7,5) = "PerlMonks";
print "$foo\n"; 
__END__
Hello, PerlMonks!
[download]

You can even write Lvalue subroutines yourself:

my $value;
sub foo : lvalue {
    $value 
}
foo() = "Bar";  
print "$value\n";
__END__
Bar
[download]

However, at least in my experience, they're only really useful in a few rare circumstances.