my $foo = "Foo";
my $bar = "Bar";
my $string = $foo . $bar;
# $string contains "FooBar".
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For concatenating the new string to the end of the existing string (appending):

my $foo = "Foo";
my $bar = "Bar";
my $foo .= $bar;
# $foo now contains "FooBar"
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For concatenating multiple strings all at once, use join. Join is (according to the Camel book) the most efficient way to concatenate many strings together. Join can be used with a delimiter, or with a null string as the delimiter, so that nothing is inserted between the strings that are joined (concatenated) together.

my $this = "This";
my $that = "That";
my $the  = "The";
my $other = "Other";
my $string = join "", $this, $that, $the, $other;
# $string now contains "ThisThatTheOther".
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Join can do a lot more than just simple concatenation. Whatever you put between the quotes will be inserted between the strings that are being joined together. Also, join can accept not just a list of scalar variables, but also an array, or a list of literal strings.