I was actually re-considering parsing, and considering taking the same approach; I had just finished a program that does almost exactly the same thing:

#!/usr/bin/perl

use strict;
use warnings FATAL => 'all';
use Data::Dumper;
use Term::ReadLine;

my %data = (
   a => 5,
   b => 1,
   c => 7,
   d => 100,
);

my $term = Term::ReadLine->new('foo');

my $expr;
while (1) {
   $expr = $term->readline('Enter expression: ');
   my $sub = compile($expr);
   print Dumper($sub->(\%data));
}

sub compile {
   my ( $expr ) = @_;
   $expr =~ s/(\w+)/\$set->{$1}/g;
   my $sub;
   my $stuff = "\$sub = sub { my (\$set) = \@_; $expr }";
   eval $stuff;
   if ( $@ ) {
      return sub { $@ };
   }
   return $sub;
}
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The more I think about it, the more I think this is sufficient.