% perl test.pl file_a
Lines:
File A line 3
File A line 4
File A line 5
File A line 6
File A line 7
File A line 8
File A line 9
File A line 10
File B line 1
File B line 2
File B line 3
File B line 4
File B line 5
File B line 6
File B line 7
File B line 8
File B line 9
File B line 10

Foos:
File A line 1
File A line 2
Use of uninitialized value at test.pl line 16.