A perl build with -DDEBUGGING can show much information during execution, for example each op being executed, and the contents of the stack after each op:

$ perl587 -Dst -e'$a = "abc"; $a .= 1'

EXECUTING...

    =>
(-e:0)  enter
    =>
(-e:0)  nextstate
    =>
(-e:1)  const(PV("abc"\0))
    =>  PV("abc"\0)
(-e:1)  gvsv(main::a)
    =>  PV("abc"\0)  UNDEF
(-e:1)  sassign
    =>  PV("abc"\0)
(-e:1)  nextstate
    =>
(-e:1)  gvsv(main::a)
    =>  PV("abc"\0)
(-e:1)  const(IV(1))
    =>  PV("abc"\0)  IV(1)
(-e:1)  concat
    =>  PV("abc1"\0)
(-e:1)  leave
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Dave.