Or maybe like this:

while(my ($param, $rx) = each %laundry) {
    local $_ = $cgi->param($param)
        or die "Parameter $param not present\n";

    ($laundered_params{$param}) = /$rx/;
        or die "Invalid input for $param\n";
}
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Note that both snippets will behave in probably undesired ways if any of the parameters have multiple values.

Makeshifts last the longest.