String concatenation

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

In string concatenation which is preferred?

$string = $var1 . $var2 . $var3;

$string = "${var1}${var2}${var3}";

$string = "$var1$var2$var3";

$string = join('', $var1,$var2,$var3);
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