my %chemElements = qw( Cobalt 58.93 Co 58.93 Copper 63.55 Cu 63.55 Curium 247 Cm 247 ); 
print "$chemElements{$ARGV[0]}\n";

my $file = 'PATH\TO\data.txt';  
 
open FILE, "$file" or die "Can't open file for read. $!\n"; 
flock FILE, 1; 
while( <FILE> ) 
{ 
my @members = split ','; 
  if( $members[0] eq $ARGV[0] || $members[1] eq $ARGV[0]  ) 
  { 
    print "$members[2]\n";  
    last; 
  } 
} 
close FILE;

$ ./chem H2O 
H2O = 18.01 
$ ./chem H2O H2SO4 
H2O = 18.01 
H2SO4 = 98.05 
$ ./chem H2O H2SO4 HCl 
H2O = 18.01 
H2SO4 = 98.05 
HCl = 36.46

#!/usr/bin/perl 
                                                                                 
use strict; 
use warnings; 
                                                                                 
my %chem; 
while (<DATA>) { 
  chomp; 
  my ($n, $s, $w) = split /,/; 
  $chem{$s} = $w; 
} 
                                                                                 
foreach (@ARGV) { 
  my $weight = 0; 
  my @parts = /([A-Z][a-z]?)(\d*)/g; 
                                                                                 
  while (my ($elem, $num) = splice @parts, 0, 2) { 
    next unless $chem{$elem}; 
    $num ||= 1; 
                                                                                 
    $weight += $chem{$elem} * $num; 
  } 
  print "$_ = $weight\n"; 
} 
                                                                                 
__END__ 
Actinium,Ac,227.00 
Aluminium,Al,26.98 
Americium,Am,243.00 
Antimony,Sb,121.76 
Argon,Ar,39.95 
Arsenic,As,74.92 
Astatine,At,210.00 
Barium,Ba,137.33 
Berkelium,Bk,247.00 
Beryllium,Be,9.01 
Bismuth,Bi,208.98 
Bohrium,Bh,264.00 
Boron,B,10.81 
Bromine,Br,79.90 
...etc...