The problem is here:

my ($on, $off, $count) = $_; # contains args
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You're assigning a scalar ($_) to a list. That will leave $off and $count as undef, which is what's causing the error. (Even $on can be undefined, if $_ is.)

The fix depends on what's in $_: whether you need to split it, or use a different variable (maybe @_), or ... .

Update
Added some clarification