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while ( my ( $k, $v ) = each %opts ) {
    for ($k) {
...
        else        { die "*** BUG: no handler for -$k.\n"; }
    }
}

while ( my ( $k, $v ) = each %opts ) {
    state %dispatch = (
...
    die "*** BUG: no handler for -$k.\n" if !exists $dispatch{$k};
    &{$dispatch{$k}};
}