IIRC from way back when
diotalevi made Enbugger, perl compiles differently when run under the debugger, and Enbugger has to rewrite the optree to replace some normal ops with debug flavors. If you can't get the -d flag set initially, I think Enbugger is the only way.
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A math joke: r = | |csc(θ)|+|sec(θ)| |-| |csc(θ)|-|sec(θ)| |