When you change units like that, in some parts of your code it might mean seconds, in some minutes. Or perhaps it was minutes and now it is hours. And you can't even clarify what the units are with a good name name because now the name changes at different points in your code.

I only consider this kind of side effect acceptable when the variable, array, etc that is being modified was just assigned. Like this:

my @minutes = @seconds;
$_ /= 60 for @minutes;
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The two fundamental principles that I would cite here, Side-effects make code harder to understand. and Variables which can't be given simple descriptive names are not well thought-through.

So, if the logic of the program allows you to avoid naming the seconds prior to the convertion, you would find that acceptable?

my @minutes = map{ $_/60 } source_of_seconds();
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Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!

Not only acceptable, but preferable.