my %symtab;

while (<FILE>) {
   chomp;
   if (/^(\w+)\|(\d+)$/) {
      $symtab{$1} = $2;
   } else {
      # Error
   }
}

print($symtab{A} + $symtab{B}, $/);
print($symtab{A} + $symtab{B} + $symtab{C}, $/);
print($symtab{C} + $symtab{A} + $symtab{D}, $/);
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Like this, or do you need to parse the equations too? Update: Here's a version that parses sums too:

my %symtab;

while (<DATA>) {
   chomp;
   if (/^(\w+)\|(\d+)$/) {
      $symtab{$1} = $2;
   } else {
      # Error
   }
}

foreach (
   'A+B',
   'A+B+C',
   'C+A+D',
) {
   my $sum = 0;

   foreach (split(/\+/, $_)) {
      my $ident = $_;
      $ident =~ s/^\s+//;
      $ident =~ s/\s+$//;
      if (exists($symtab{$ident})) {
         $sum += $symtab{$ident};
      } else {
         # Error
      }
   }

   print("$_=$sum$/");
}

__DATA__
A|10
B|20
C|20
D|40
[download]