this:

    package Blah;
    use strict;
    require Exporter;
    our @ISA = qw/Exporter/;
    our @EXPORT_OK = qw/authenticate/;
    # ...

or,

    package Blah;
    require Exporter;
    @ISA = qw/Exporter/;
    @EXPORT_OK = qw/authenticate/;
    use strict;
    # ...

or, 

    package Blah;
    use strict;
    use vars qw/@ISA @EXPORT_OK/;
    require Exporter;
    @ISA = qw/Exporter/;
    @EXPORT_OK = qw/authenticate/;
    # ...