Hashes accept barewords in their braces and treat them as the keys. Perl interprets your line

delete $ENV{_}
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just like it treats the next line:

print "$ENV{FOO}\n";
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There is little way around that because the second usage is quite common. You could potentially set up some policy for Perl::Critic to prohibit/flag usage of bare hash keys that do not start with [A-Za-z] or something like that. Perl more or less considers everything matching /\s*\w+\s*/ as a valid bareword hash key (he says without checking).