|XP is just a number|
As corion pointed out the problem, here I am just adding extra information for you.
There are two forms of the open() function in Perl 5. The modern version takes three arguments: the filehandle to open or vivify, the mode of the filehandle, and the name of the file.
The legacy version has two arguments, only the filehandle and the name of the file. The mode of the file comes from the filename; if the filename starts (or ends) with any of several special characters, open() parses them off and uses them.
If you accidentally use a filename with those special characters with the two-arg form of open(), your code will not behave as you expect. This is especially a problem if you're not careful about sanitizing user input, and if any user input ever becomes part of a filename. Consider:
While this code appears to open $filename for writing, an insecure $filename could start with > to force appending mode, or - to open STDOUT (though I suspect you have to work really hard to force this). Likewise, code without any explicit mode in the second and final parameter is susceptible to any special mode characters.
Extracting file modes into a separate parameter to this function prevents Perl from parsing the filename at all and removes the possibility for this unintentional behavior. As Damian Conway has mentioned, using a separate file mode parameter also makes the intention of the code clearer:
The modern version of this code is safer and clearer, and it's been available since Perl 5.6.0 sourced from http://modernperlbooks.com/mt/2010/04/three-arg-open-migrating-to-modern-perl.html
change the line my @nets = <IN2>; to chomp (@nets = (<IN2>));
All is well. I learn by answering your questions...