The code is simple and no hidden implementation. always undefined is set as 2 in our environment so it prevents the <UNDEFINED>.

I keep the if statement in both single line as well as block structure

 s mr=15824,vs="EV1"
 s x=$S(^pmr(1):$NA(^pmr),1:"")
 if $d(@x@(mr,vs)) {
    s ^zxq($now())=@x@(mr,vs)
 }
 q

I set $T to 0 before the if statement. After the syntax error occurred, I checked the value of $T. It seems that the interpreter doesn’t differentiate between a block of code and a single line — it still executes the subsequent lines inside the condition

I modified the post slightly. I ran a routine via the terminal, and as expected, it threw a <SYNTAX> error because the variable x is undefined. Then I continued the program, and now both the single-line if statement and the block if statement (with braces) set all the global variable. I thought that block of code wouldn’t be executed

 if $D(@x@(a,b,c)) s ^zz1=1,^x=1,^y=2,^xx=1
 ;
 ;
 if $D(@x@(a,b,c)) {
     set ^zz1=1212
     set ^dd=1
     set ^fg=2
 }