Not a serious proposal, but another example of TIMTOWTDI:
You may create an array that holds an alias to $a as the second element and interpolate it into a string.

#!/usr/bin/perl

use v5.16;
use warnings;

my $a = '';
local our @str;
sub {*str = \@_}->('Hello', $a);
$a = 'Kevin';
say "@str";
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