###WRONG      
if ($chemical = $periodicTable[$i][1]) { #always activates

###RIGHT
if ($chemical == $periodicTable[$i][1]) { 

##</code><code>##

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

)