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my %periodic_table = ( 1 => {symbol=>'H', name => 'Hydrogen', weight =
+> 1},
2 => {symbol=>'He', name => 'Helium', weight => 4}, )

<c>
my %periodic_table = 
...
      He => {atom_num => 2, name => 'Helium',   weight => 4}, 
      # ...
    );

my ($atom_nr, $elmnt_name, $atom_weight) = @{$periodic_table{$chem_sym
+bol}}{qw /atom_num name weight/};

my $atom_nr = $periodic_table{$chem_symbol}{atom_num};
my $name = $periodic_table{$chem_symbol}{name};
my $atom_weight = = $periodic_table{$chem_symbol}{weight};