It's unfortunate that operators are so hard to search for. Search engines really ought to point you to perlintro, or perlop, both of which discuss the dot and the dot-assignment operators.

The dot operator catenates two strings:

my $foo = "bar" . "baz";
print $foo, "\n";
[download]

...prints "barbaz". The .= operator does the same thing, but assigns the new string to the scalar on the left.

my $foo = "bar";
$foo .= "baz";
print $foo, "\n";
[download]

...also prints "barbaz".