use strict;
use warnings;

my %data = ( x => 5, y => 7, z => 2 );

my @formulae = ( 
	'x + y',
	'z + 3',
	'x ** z',
	'a + x',
	'x /= y',
	'x - x'
);

for my $formula (@formulae) {
	print "Processing formula $formula\n\t";
	my @error = ();
	while ($formula =~ s|([a-z]+)|$data{$1}//' '|e) {
		push @error, $1 unless exists $data{$1};
	}
	if (@error) {
		print "Error: unknown variable(s) @error\n\n";
	} else {
		my $result = eval($formula);
		if( defined $result) {
			print "Result: $formula = $result\n\n";
		} else {
			print "Syntax error in $formula\n\n";
		}
	}
}