my %parsers = (
    'Ethernet' => sub {
        my $chunk = shift;
        $chunk =~ tr/\n/ /;
        return "ETHERNET: $chunk";
    },
    'Gigabit' => sub {
        my $chunk = shift;
        $chunk =~ tr/\n/ /;
        return "GIGABIT: $chunk";
    }
);

$/ = "!\n";
$\ = "\n";
while ( <DATA> ) {
    chomp;
    if ( /^interface (.*)/ && $1 && exists $parsers{$1} ) {
        print $parsers{$1}( $_ );
    } elsif ( /^system (.*)/ ) {
        # blah...
    }
}