perl script multiline replace without slurping entire file

sub deskriptoren {

open my $in, "<export.txt" or die "Cannot find 'export.txt': $!\n";
open my $out, ">LIDOS_export_erweitert.txt" or die "Cannot create 'LIDOS_export_erweitert.txt': $!\n";

print "\n\tExpand Deskriptor-Chains.\n";

while(my $line = <$in>) {
    print $out $line;

    if( $line =~ m{^\+Deskriptoren:} ) {
        my($section, $next_line) = _read_to_next_section($in);

        print $out _munge_description($section);

        print $out $next_line;
        }
    }
close $in;
close $out;
undef $in;
undef $out;
print "\t\tCompleted.\n\n";


sub _read_to_next_section {
        my $in = shift;

        my $section = '';
        my $line = '';
        while( $line = <$in> ) {
            last if $line =~ /^ \+ /x;
            $section .= $line;
            }
        # When reading the last section, there might not be a next line
        # resulting in $line begin undefined.
        $line = '' if !defined $line;
        return($section, $line);
        }           # End of "_read_to_next_section"-subroutine


sub _munge_description {
    my $description = shift;

    $C+=$description =~ s#foo#bar#gm;
    $C+=$description =~ s#((?:-(?!PRC).+\n)*)-(Cultural Revolution|bla|blo|blu)\n((?:-(?!PRC).+\n)*)#${1}-PRC\n-${2}\n${3}#;
    # a _lot_ more similar regexes.

    return $description;
}           # End of "munge_description"-subroutine
}           # End of "Deskriptoren"-subroutine