~/MyPerl/
    |
    |__ mycode.pl
    |__ subdir
          |____ data.pri
          |____ data.sec
 

##</code><code>##

# mycode.pl

my $file     = "/files/data.pri";

my $name     =  (split(/[\/\.]/,$file))[2];
my $file_sec = "/files/$name.sec";

my $result = some_function($file,$file_sec);