I'm not sure where you are getting

b $mysubref
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from. The doc I see says this, which seems to be the behaviour you are triggering:

b [line] [condition]
    Set a breakpoint before the given line.  If a condition is
    specified, it's evaluated each time the statement is
    reached: a breakpoint is taken only if the condition is
    true.  Breakpoints may only be set on lines that begin an
    executable statement.  Conditions don't use "if":
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That is, you are setting a break point at the current line (because you specify no line) if $mysubref is true.