Reference Variables

In Perl, you can't give a variable a specific type to ensure that it's able to store only particular kinds of values (integer, string, reference, and so on). That's usually not a problem, because Perl's automatic type conversions paper over most of the cracks very neatly ^[13]

Except when it comes to references.

It's an all-too-common mistake to put a reference into a scalar, and then subsequently forget to use the all-important dereferencing arrow:

sub pad_str {
    my ($text, $opts) = @_;

    my $gap   = $opts{cols} - length $text;        # Oops! Should be: opts->{cols}
    my $left  = $opts{centred} ? int($gap/2) : 0;  # Should be: opts->{centred}
    my $right = $gap - $left;

    return $SPACE x $left . $text . $SPACE x $right;
}