Your main loop could look something like:

while ( <$IN> ) {
    print $LOG $_
        if ( s/t/tt/ig );
    print $OUT $_;
}
[download]

A global substitute, as above, isn't going to cause an infinite loop. It'll change

I would like to substitute globally

to

I would like tto substtittutte globally

In-place editing isn't always appropriate, so you may have to write a temp file and rename it once you've processed the input.

Why don't you have a go and come back here if you get stuck?

Good luck!

But this would disable multi-line substitutions. So I need a way to replace in a text buffer that is loaded from a file.
I would like to use the same feature also for already loaded files in a editor window.
I also create a backup of the file before applying the changes so that they can be undone latter.

In which case, I'd slurp the file in, do the substitutions and write it back out to a temp file. Then I'd run diff on the original & temp files.

Since you mention changed lines and line numbers it appears that you expect changes to be limited to single lines, as opposed to changes spanning multiple lines. If so, you'd be much better off looping over the file line by line and logging changes as they happen. Note the $. variable, which will give you the line number.

Anno

No, but the line where the substitution starts would be enough. So going line by line isn't really an option since this would disable multi-line substitutions. Isn't there a cool trick using the e modifier for substitutions?

You could try to push the value of pos onto an array.

$str =~ s{$pattern}{push @positions, pos $str; $substitution}eg

I never worked with pos so I don't know if that helps you.

---

Perl 6 in German -- Difficult Sudoku

How do you expect /e could help? It only changes the behavior of the substitution side of s///.

One possibility is to step through substrings of the file content, starting at each new line. Here is a sketch:

my $content = do { local $/; <DATA> };

my ( $ln, $start) = ( 1, 0);

while ( $start < length $content ) {
    if ( substr( $content, $start) =~ s/.../.../ ) {
        print "line $ln changed\n";
    }
    $start = 1 + index( $content, "\n", $start);
    ++ $ln;
}
__DATA__
aaa
bbb
ccc
ddd
eee
[download]

Anno

while (my $line = <$infile>){
    if ($line =~ s/T/TT/g){
        # log $line and $. (line number)
    }
    print $new_file $line;
}
# overwrite old file with new file here
[download]

But this would disable multi-line substitutions. So I need a way to replace in a text buffer that is loaded from a file.
I would like to use the same feature also for already loaded files in a editor window.