-p processes the file one line at a time, while both of your regexp rely on matching two lines at a time. There are other problems with your regexps, but that's the biggest. That can be solved by working with more than one line at a time. 

my $file = do { local $/; <> };
1 while $file =~ s/^([ ]{4}[^\n]+)\n[ ]{4}/$1/m;
print($file);

[download]

Or better yet: 

my $file = do { local $/; <> };
1 while $file =~ s/^(([ ]+)(?![ ])[^\n]+)\n\2(?![ ])/$1/m;
print($file);

[download]

Note: The second will unwrap all pragraphs, no matter how far they are indented.

Note: Both work with and without -i.

Note: 1 while s///; is used in order to repeatedly match the same line. s///g; won't work when the paragraph is more than two lines long.

In reply to Re: replace \n with space to join indented lines by ikegami
in thread replace \n with space to join indented lines by olivier

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