Documentation for infix .= assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix .= §

Calls the right-side method on the value in the left-side container, replacing the resulting value in the left-side container.

In most cases, this behaves identically to the postfix mutator, but the precedence is lower:

my $a = -5;
say ++$a.=abs;
# OUTPUT: «6␤» 

say ++$a .= abs;
# OUTPUT: «Cannot modify an immutable Int␤
#           in block <unit> at <tmp> line 1␤␤»