$/ = "";
my $last_prefix = my $last_target = '';
while (<>) {
    if ( /^abc/ ) {
        print $last_prefix.$last_target if ( $last_target );
        $last_prefix = $_;
        $last_target = '';
    }
    elsif ( /^def/ ) {
        $last_target = $_;
    }
    elsif ( $last_target ) {
        $last_target .= $_;
    }
    elsif ( $last_prefix ) {
        $last_prefix .= $_;
    }
}