John,
Why the EXECUTE?
You could just have easily done:
48110 A=SYS("/data/DATAMpvx/emailer.pl '"+WHOFM$+"' '"+EMAIL$+"' '"+ATTACHMENT$+"' '"+SUBJ$+"'")
Then just test A... or use the SYS function as the subject of an IF.
The extra EXECUTE seems redundant and forces the system to compile the command prior execution slowing things down.
Side note: I trust your logic makes sure all the fields cannot contain apostrophes -- for example you need to make sure the Subject line is not
"Sorry, I can't come out and play"
The Apostrophe in "can't" will mess up your logic regardless of the EXECUTE or not.