Your problem is that you didn't localize $_ in the sub. As a result, the $_ from the outside world is changed. And since parameters are passed by reference, your parameter is changed after you passed it in. "$_" passes a copy, so that one doesn't change.

As a fix, which IMO is much less interesting than the phenomenon we just witnessed, you can localize the change by

sub foo
{
    while (@_) { local $_ = shift; $params->{$_} = shift; }
    print "$_: $params->{$_}\n", for keys %$params;
}
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or

sub foo
{
    local $_;
    while (@_) { $_ = shift; $params->{$_} = shift; }
    print "$_: $params->{$_}\n", for keys %$params;
}
[download]