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# match a line that only contains one new line
if ($line =~ /^\n$/) {

# match a line that only contains one or more whitespace
if ($line =~ /^\s+$/) {

foreach (@blah) {
   if ($foo) {
...
      # other stuff
   }
}

# open filehandles ...

...

    print OUTPUT $line if $s and $line !~ /^\s+$/;
}