b subname [condition]
                   Set a breakpoint before the first line of the named
+ subroutine.  subname may be a variable containing a code
                   reference (in this case condition is not supported)
+.

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The perldoc perldebug has a bit more information. Note the word "named". Seems to imply that you can't do that with anonymous subroutines.

In reply to Re: b $var by jethro
in thread b $var by Anonymous Monk

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