useage of => in join in perl

useage of => in join in perl

The Perl Programming contain a piece of code like this:
for $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
say "$family: ", join(", " => sort @{ $HoA{$family} });
}
What I'd like to know is what does the "=>" mean in the join function?