This is documented in pos. That page contains links providing additional information.

For future reference, you can find documentation for Perl's functions via "Perl functions A-Z".

Within a regular expression, \G, used in conjunction with the /g modifier, matches at the position in the target string where the previous match left off. pos also contains the position where the previous match ended. They are tightly related; setting pos to a number makes \G match at that position within the target string.

It has various other side-effects with respect to how m//g work, so see perlretut for a more thorough explanation.

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.