Fix for multiple constant declarations

Before:
```
~/constant-fromglobal[master]$ PERL5LIB=lib perl -Mstrict -MData::Dumper
use Constant::FromGlobal {env => 1}, "DSN", MAX_FOO => {int => 1, default => 3};
print Dumper(DSN, MAX_FOO);
Bareword "MAX_FOO" not allowed while "strict subs" in use at - line 2.
Execution of - aborted due to compilation errors.
~/constant-fromglobal[master]$ 
```
After:
```
~/constant-fromglobal[master]$ PERL5LIB=lib perl -Mstrict -MData::Dumper
use Constant::FromGlobal {env => 1}, "DSN", MAX_FOO => {int => 1, default => 3};
print Dumper(DSN, MAX_FOO);
$VAR1 = undef;
$VAR2 = 3;
~/constant-fromglobal[master]$
```