I understood that your need is to read a file, line by line, and if the line is not blank, write the line to the output file. Did I understand that correctly?

My code snippet, as you have quoted above, will:

• Read one line from the input file
• Remove a potential newline from the end of the line
• Look if the line is now empty (zero length)
• If empty, read next line
• If not empty, write the line and a newline to the output file and then read next line from the input file
• Repeat until no more lines in input file

What part is not working as expected?

Update:
I admit, I missed the words:
"...specifically for finding and replacing all occurences of a string..."

Of course, you're right. ;-) That's what I get for answering a question while on the run to go home. ;-)

while(<IN_FILE>)
   s/from/to/g;
   print OUT_FILE;
   }
[download]

should do it.

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Everything went worng, just as foreseen.