D:\tmp>perl -MO=Xref,-r,-d,-DO tst_b_xref2.pl
OP (0x163e650) enter
COP (0x10fcdd8) nextstate
OP (0x10fce38) padsv
tst_b_xref2.pl   (main)           3 (lexical)       $ sth              intro
COP (0x163e698) nextstate
LOOP (0x163e740) enterloop
OP (0x16579c8) pushmark
OP (0x1657a50) pushmark
PADOP (0x1657b48) gvsv
tst_b_xref2.pl   (main)           5 main            $ "                used
OP (0x1657ad0) padav
tst_b_xref2.pl   (main)           5 (lexical)       @ row              used  #:used?
LISTOP (0x1657a88) join
LISTOP (0x1657a08) print
OP (0x163e870) unstack
OP (0x1657bd0) pushmark
OP (0x1657d60) pushmark
OP (0x10fcd18) padsv
tst_b_xref2.pl   (main)           5 (lexical)       $ sth              used     
METHOP (0x1657d18) method_named
UNOP (0x1657cd0) entersub
tst_b_xref2.pl   (main)           5 (lexical)       $ sth              subused
OP (0x1657c50) pushmark
OP (0x10fcd58) padav
tst_b_xref2.pl   (main)           5 (lexical)       @ row              intro #:5?intro?
BINOP (0x1657b88) aassign
LOGOP (0x163e7e0) and
BINOP (0x163e6f8) leaveloop
LISTOP (0x10fcd90) leave
tst_b_xref2.pl syntax OK