# File1.pm
package File1;
use File2;

sub foo {
  File2::foo(@_);
  other();
  code();
  appended();
  here();
}

1;

# File2.pm
package File2;

sub foo {
  original();
  code();
  here();
}

1;

# File 3
use File1;
File1::foo(); # this will call File2::foo() behind the scenes