use strict;
my $str = "foobar";
my $c = MyConstants->new();

print "str = $str\n";
$str =~ s/$c->{Foo}/$c->{Bar}/;
print "str = $str\n";
print "Bar = $c->{bar}";


package MyConstants;

use constant Foo => "foo";
use constant Bar => "bar";

use Tie::Hash;
use base qw(Tie::StdHash);
use Carp qw(croak);

sub new {
    tie my %self,  $_[0];
    return bless \%self, $_[0];
}

sub FETCH {
    if( $_[0]->can($_[1]) ) {
        goto &{$_[1]};
    } else {
        croak("Undefined constant \"$_[1]\" called");
    }
}

##</code><code>##

str = foobar
str = barbar
Undefined constant "bar" called at /tmp/test.pl line 8