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foreach (keys %insitution_table)
{
     $institution_table{$_}{ages}[$age/5]++;
}

my %valid_institutions = map { $_ => $_ } (
      $hospital1,
...
          || "unaffiliated";

$institution_table{$key}{count}++;

my %foo = map { $_ => 1 } qw[ foo bar baz ];

my $foo = 'foo';
print "\$foo is in the list\n" if ($foo{$foo});