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";
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...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";
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...also prints "barbaz".

$foo .= $bar is shorthand for $foo = $foo . $bar. The appropriate links for the dot operator have been given above.