take the following example-
WHEN 1 PERFORM PARA-1
WHEN 2 PERFORM PARA-2
WHEN 3 PERFORM PARA-3
depending on the value of VAR1 it execute para-1 to para-3.
here WHEN 1 PERFORM PARA-1 is equivallent to the statement
IF VAR1 = 1
now see the following
EVALUATE VAR1 ALSO VAR2
WHEN 1 ALSO 2 PERFORM PARA-1
WHEN 1 ALSO 3 PERFORM PARA-2
here to perfrom para -1 var1 and var2 must contain value 1 and 2.
ie the statement is equivallent to
IF VAR1 = 1 AND VAR2 = 2
i think the above discussion will clear your concept