Variables::ProhibitUnusedVariables false positive with .= in initial assignment

[epa]

my $x .= '';
my $y .= <<END
$x
END
;
print $y;

This gives a false positive

"$x" is declared but not used [Variables::ProhibitUnusedVariables]

Of course the initial assignment with .= is fishy, and there is a separate policy for that, but the variable is not unused. The heredoc seems to be necessary to trigger the bug.