use constant RX_COMMENT => qr{ ... }xms;
...
...
sub is_valid {
  
    my ($string,
        ) = @_;
        
    return
        $string =~ RX_COMMENT      ? 0 :
        $string =~ RX_BLANK_STRING ? 0 :
        $string =~ RX_VALID_LINE   ? 1 :
        die "invalid: '$string'"   # default
        ;
  
    }
...
...
LINE:
while (my $line = <$fh>) {

    next LINE unless is_valid($line);

    do_something_with_known_valid($line);
    do_something_else_with($line);
    ...

    }  # end while LINE