Interestingly the filehandle _ gets stringified but a conventional file handle (stringified as a GLOB) can be used as a hash key:

open(my $fh, $0) or die $!;
$gash{$fh} = 'some value';

stat($0);
$gash{*_}  = 'some value';
print Dumper(\%gash);

$VAR1 = {
          '*main::_' => 'some value',
          'GLOB(0x18ab6ac)' => 'some value'
        };
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Of course just because it is legal does not mean it is necessarily a good idea.
use strict; use warnings; can never catch all errors, just that you will catch more with them than without.