If ModA uses ModB and ModB uses ModA, you'll have problems exporting symbols. The best way I've found is to do:

# ModA.pm

use strict;
use warnings;

package ModA;

BEGIN {
   our @ISA = qw( Exporter );
   our @EXPORT_OK = qw( ... );
   require Exporter;
}

use This;
use ModB;
use That;

...

1;
[download]

# ModB.pm

use strict;
use warnings;

package ModB;

BEGIN {
   our @ISA = qw( Exporter );
   our @EXPORT_OK = qw( ... );
   require Exporter;
}

use This;
use ModA;
use That;

...

1;
[download]

Otherwise, things usually work out fine.