well, eval can do this, but comes with the usual caveats that a) you have to trust the strings you give it and b) there's probably a better approach ..

foreach (qw/hasha hashb hashc/) {
  @keys = keys eval('%' . $_);
  print "There are " . scalar(@keys) . " keys here for '$_'\n";
}
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As for a better approach, first thought is that those hashes should originally be constructed as hashrefs and in a hash...

my %big_data = (
  hasha => \%hasha,
  hashb => \%hashb,
  hashc => \%hashc,
);

foreach (qw/hasha hashb hashc/) {
  @keys = keys %{$big_data{$_}};
  print "There are " . scalar(@keys) . " keys here for '$_'\n";
}
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