sub test {
    for (my $i = 0; $i < @_; $i += 2) {
        if ($_[$i] eq 'a') {
            $_[$i+1] = 'found a';
        }
    }
}

my $a = 'not found';

test b => 3, a => $a;
print "$a\n";
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But beware, chaning your caller's arguments is evil, and usually produces code that's hard to understand and read.