I think this is what you wanted :

#!/usr/bin/perl

use strict;
use warnings;

sub foo
{
    my %params = @_;
    print "$_: $params{$_}\n", for keys %params;
}

my $word = shift @ARGV || "world";

foo ( hello => $word );
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Do not use $_ as a temporary variable unless essential.

Declaring and naming the variable catches a lot of issues and solves (for example) the problem you came across as well as improving readability.