The redundancy can be eliminated:

#!/usr/bin/perl

my $MaxLineLength = 8060;
$search = '<COLUMNS>';
$replace = '<DATA>';
$searchEnd = '</COLUMNS>';
$replaceEnd = '</DATA>';
$fileContents = '';

my $linePrefix = '';

while (<STDIN>) {
   chomp; # Remove newline
   if (/(^<DATA>\t).*(<\/DATA>$)/) {
      next if length() > $MaxLineLength;
   } elsif (/(^<COLUMNS>\t).*(<\/COLUMNS>$)/) {
      s/$search/$replace/g;
      s/$searchEnd/$replaceEnd/g;
   } else {
      next;
   }

   print $linePrefix; # Empty only for first line
   print;
   $linePrefix = "\n";
}
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