I have read many forums

Maybe just the documentation would have been enough. Since you can write pseudocode, we'll assume you are a programmer and are familiar with the usual constructs.

For if ... then ... else ... end if see Compound Statements.

For LINE contains "SUM" see index or perlre.

For remove last two characters in line you have lots of options, including substr, chop, the s/// operator, etc.

Note that there are many Tutorials right here in the Monastery to help you get started. Enjoy.