How about: 
do {
  $n = chomp;
  print;
  $_ = <>;
  print $/ x $n unless s/^;//;
} while defined($_);

[download]
Works with empty files and files that begin with a ;

Update: This is just another way to think about the problem. You don't have to start every file reading loop with while (<>).

Update 2: Changed unless /^;/ to unless s/^;// which addresses concerns about it not working with consecutive lines beginning with a semi-colon.

In reply to Re: append line to previous line in txt file by pc88mxer
in thread append line to previous line in txt file by mmittiga17

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