I'm completely new to COBOL, and I'm wondering:
There seems to be no difference between
What does the dot at the end of a line actually do?
When should I use/avoid it?
The period ends the "sentence." It can have an effect on your logic. Consider...
IF A = B PERFORM 100-DO SET I-AM-DONE TO TRUE.
IF A = B PERFORM 100-DO. SET I-AM-DONE TO TRUE
The period ends the IF in both examples. In the first, the I-AM-DONE 88-level is set conditionally, in the second it is set unconditionally.
Many people prefer to use explicit scope terminators and use only a single period, often on a physical line by itself to make it stand out, to end a paragraph.