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#!/usr/bin/perl 

...

warn Dumper [ grep {$$h{$_}} qw/a b c x/ ];      # <-- no side effect
warn Dumper [ keys %$h ];

C:/Perl_524/bin\perl.exe  d:/tmp/pm/grep_autovivify.pl 
$VAR1 = [
...
          'c',
          'b'
        ];