It's one example of the generalisation of operators like +=: $x OP= $y; is the same as $x = $x OP $y;, where OP can be most of the common binary operators (meaning operators with 2 arguments, one on each side of the operator). So $x .= $y; is the same as $x = $x . $y;

See Assignment Operators in perlop.

In reply to Re: What does ".=" mean? by bart
in thread What does ".=" mean? by anaconda_wly

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