As always, TMTOWTDI - From some of my code ...

use Carp;

sub example {
    my %options = (
        'DEBUG'     =>  0,
        'TMPL_PATH' =>  undef
    );
    my %args = ( %options, @_ );
    foreach ( keys %args ) {
        croak( 'Unknown argument passed to subroutine - ', $_ )
            unless exists $options{$_};
    }

    .
    .

}
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This code makes use of a second hash for the definition of default values for arguments and incorporates a check for unknown arguments by comparing the keys of the merged hash %args with that of the default values (%options).