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       -Idirectory
            may  be  used  in  conjunction  with -P to tell the C prep
+rocessor
...
            with  any  words  recognized  by  the C preprocessor such 
+as "if",
            "else" or "define".)

#define ADD(x,y)  x + y

my $r = ADD( getPfromSomewhere(), getQfromSomewhere() );
...
my $s = ADD( geta(), getb() );

# 1 "macro.pl"
# 1 "<command-line>"
...
...
my $s = geta() + getb();