$/ = "!\n";
while ( <DATA> ) {
    chomp;
    if ( /^interface (.*)/ ) {
        if ( $1 eq 'Ethernet' ) {
            parse_ethernet( $_);
        } elsif ( $1 eq 'Gigabit' ) {
            parse_gigabit( $_);
        }
    } elsif ( /^system (.*)/ ) {
        # blah...
    }
}

sub parse_gigabit {
    my $chunk = shift;
    $chunk =~ tr/\n/ /;
    print "GIGABIT: $chunk\n";
}

sub parse_ethernet {
    my $chunk = shift;
    $chunk =~ tr/\n/ /;
    print "ETHERNET: $chunk\n";
}

__DATA__
!
interface Ethernet
 blah...
 blah...
!
interface Gigabit
 blah...
 blah...
!
interface Ethernet
 blah...
!