To Perl-ize this wonderful Lisp solution *grins*

sub flatten {
    return unless @_;
    my $val = shift;

    return flatten(@$val, @_) if UNIVERSAL::isa($val, 'ARRAY');

    return $val, flatten(@_);
}
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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.