my $str = 'dfsldjf(djfls)jfslfk kfds\nfjsldf';

output: dfsldjf_djfls_jfslfk_kfds fjsldf

Did you really mean to include a newline character (single quotes prevent interpolation)?

use warnings;
use strict;

my $str = 'dfsldjf(djfls)jfslfk kfds\nfjsldf';
$str =~ s/[\s()]/_/g;
$str =~ s/\\n/ /g;
print "$str\n";

__END__

dfsldjf_djfls_jfslfk_kfds fjsldf
[download]

well I have a file a lines and there is \n between lines. In order to send an example of what I have in one string, I included \n in the string. Otherwise, it's like

HeadeRdfsldjf(djfls)jfslfk kfd

lfjslfjsflk

dkflskfs

HeadeRRdfsldjf(djfls)jfslfk kfd

lfjslfjsflk

dkflskfs

in the header line, ()\s should be replaced by _, \n at the end should be replaced by \s, on the following lines without Header, \n should be removed except the last one so that it won't be concatenated with the next header line:

HeadeRdfsldjf_djfls_jfslfk_kfd lfjslfjsflkdkflskfs

HeadeRdfsldjf_djfls_jfslfk_kfd lfjslfjsflkdkflskfs

=~ s/\\n/ /g; doesn't work for me. I don't get space instead of \n.

So finally, $str =~ s/\n/ /g; removes \n. Does it remove EOF, too? If so, how to avoid?