Actually, it's not a backtick thing; that was just the first thing I tried; when I run your code I also get the desired results. Specifically:

# lll.pl
sub longestLineLength {
  my $longestLen = undef;
  for my $l ( @_ ) {
    if ( length($l) > $longestLen ) {
      $longestLen = length($l);
    }
  }
  return $longestLen;
}

1;  # propitiatory offering for require
__END__

# test03.pl
use strict;
use warnings;

require "lll.pl";

open (TOP, "COLUMNS=1000 /usr/bin/top -bcS -n 1 2>err.txt|")
  || die "open pipe from top failed $!";
print longestLineLength( <TOP> ) . "\n";
__END__
% perl test03.pl
208
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