Given Two.pl as written, One.pl would look like this:

package MyPerl;
sub Main_Fn
{
  Second_Pak::ProcessData();
}
1;
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You can access variables and subroutines in another package simply by prepending the package name.

Alternatively, you could switch into the other package inside the block:

package MyPerl;
sub Main_Fn
{
  package Second_Pak;
  ProcessData();
}
1;
[download]

That can be useful if you want to access a bunch of stuff from the other package.