package Thingy;

sub new {
  my ($class, $line) = @_;
  my $thingy = bless {}, $class;
  my $pattern = $thingy->pattern;
  ...
  if ($line =~ m/$pattern/) {
     ...
  }
  return $thingy;
}

sub pattern { die "subclass should override me" };

...

package Rubber;
@ISA = qw(Thingy);

sub pattern { q(...) };