Haha...yes I also use a few includes to hide some ugliness. Here's my solution.
:SET &STRING# = 'JIM1.TXT, JIM2.TXT, JIM3.TXT, JIM4.TXT'
:SET &MULT# = STR_FIND(&STRING#, ',')
:SET &MULT# = FORMAT(&MULT#)
!
:IF &MULT# > '0'
: SET &BEG# = '1'
: SET &COMMA# = 'Y'
: WHILE &COMMA# = 'Y'
: SET &END# = STR_FIND(&STRING#,',',&BEG#)
: IF &END# > '0'
: SET &COMMA# = 'Y'
: SET &LGTH# = SUB(&END#,&BEG#)
: SET &FILE# = STR_CUT(&STRING#,&BEG#,&LGTH#)
: SET &FILE# = STR_TRIM(&FILE#)
: PUT_VAR VARA.STRING_SPLIT, &FILE#,
: PRINT "&FILE#"
: ELSE
: SET &COMMA# = 'N'
: SET &FILE# = STR_CUT(&STRING#,&BEG#)
: SET &FILE# = STR_TRIM(&FILE#)
: PUT_VAR VARA.STRING_SPLIT, &FILE#,
: PRINT "&FILE#"
: ENDIF
: PRINT "COMMA FOUND? &COMMA#"
: SET &FILE# = STR_TRIM(&FILE#)
: SET &BEG# = ADD(&END#,1)
: SET &BEG# = FORMAT(&BEG#)
: PRINT &BEG#
: ENDWHILE
:ELSE
: SET &FILE# = STR_TRIM(&STRING#)
: PUT_VAR VARA.STRING_SPLIT, &FILE#,
: PRINT "FILE#"
:ENDIF