Thanks for the guidance. I'm trying to digest it all.

Don't assign to a global variable without localising it. You just clobbered your parent's data.

I guess you mean like this:

use strict;
use warnings;
use 5.010;


sub do_stuff {
    foreach (1..5) {
        &dangerous($_);
        say "calculation in do_stuff(): ", $_ * 10;
        
    }
}

sub dangerous {
    $_ = shift;
    $_ *= 100;
    say "calculation in dangerous(): $_"
}

&do_stuff();

--output:--
calculation in dangerous(): 100
calculation in do_stuff(): 1000
calculation in dangerous(): 200
calculation in do_stuff(): 2000
calculation in dangerous(): 300
calculation in do_stuff(): 3000
calculation in dangerous(): 400
calculation in do_stuff(): 4000
calculation in dangerous(): 500
calculation in do_stuff(): 5000
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By the way, I'm calling all user defined functions using the syntax &func() because the llama 5th ed. merely says that the & prevents name clashes with perl functions, therefore beginners should use the & until they become familiar with the names of perl functions. However, from the comments I gather there is more to it than that.

sub do_stuff1 {
    local $_ = shift;
    chomp;
    say;
}
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or even safer:

sub do_stuff1 {
    ${ local *_ } = shift;
    chomp;
    say;
}

The latter handles the case where $_ is tied or otherwise magical.

What does that * notation do, and could you give some more details why that is safer than local $_? Maybe an example?

Thanks.