The exact answer to your question is to wrap this code in a block with strict refs turned off:

{
  no strict 'refs';
  foreach (qw/hasha hashb hashc/) {
    @keys = keys %$_;
    print "There are @keys here";
  }
}
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But the better answer is a question: why are you doing this? Why not just store the data in one hash to begin with, for example $hash{a}{key1} instead of $hasha{key1}?