thanks for this. As it now issues the warning (perl v5.36.0):

I propose this variation: ($foo) = keys %{{$foo,0}}

Also, here is a test script using Test::Taint to check taintness:

#!perl -T
use strict;
use warnings;
use Test::More;
use Test::Taint;

my $foo = "ababab";
taint($foo);
tainted_ok($foo, "foo is tainted");
($foo) = keys %{{$foo,0}};
untainted_ok($foo, "foo is now untainted");

done_testing;
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bw, bliako