I use %args_in which covers the meaning nicely. Maybe %params_in would be even better.

No, parameters are expected, while arguments are passed. The hash contains arguments, never parameters.

Given foo => $bar, the parameter is "foo", and the argument is $bar, named by the name of the parameter.

In Perl 6:

sub foo ($foo, $bar) {   # $foo and $bar are *parameters*
    # But in the sub itself, they represent the *arguments*
    ...
}

foo(42, 15);                # positional arguments
foo(foo => 42, bar => 15);  # named arguments
foo(bar => 15, foo => 42);  # same thing :)
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