It is already configurable. Look a few lines further down at the import method:

sub import
{
    my ($class, @args) = @_;
    shift @args
        if @args && exists $args[0] && defined $args[0] && $args[0] =~
+ /^[\d\.\_]{1,10}$/;
    Inline->import(Java => 'DATA', CLASSPATH=>$classpath, @args);
}
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So the CLASSPATH => $classpath can be overridden by passing arguments to import a la:

use XML::Saxon::XSLT2 CLASSPATH => "/path/to/saxonb.jar";
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There is an example of providing arguments to the import method in the documentation (though not the CLASSPATH argument).

However I've not played with it much. If your application contains multiple instances of "use XML::Saxon::XSLT2" it's probably only the first one that counts. To be sure, you can supply the path on the command line like:

perl -MXML::Saxon::XSLT2=CLASSPATH,/path/to/saxonb.jar myscript.pl
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PS: patches would be appreciated for this issue (or indeed for RT#66278 which is a long running one).