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

##</code><code>##

<c>
my %periodic_table = 
    ( H  => {atom_num => 1, name => 'Hydrogen', weight => 1},
      He => {atom_num => 2, name => 'Helium',   weight => 4}, 
      # ...
    );

##</code><code>##

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

##</code><code>##

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