To get a little closer to your objective, you might try one of the following:

#!/usr/bin/perl -w
#
use strict;

my $string = "the quick brown fox jumped over the lazy dog";

# Using a regular expression
$string =~ m/(.{0,50}\s)(.*)/;
print "$1\n$2\n";

# Using rindex() and substr()
my $pos = rindex($string, ' ', 50);
print substr($string,0,$pos) . "\n" . substr($string,$pos+1) . "\n";
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The regular expression will break the line on any whitespace (space, tab, etc.) but rindex will break only on a space character.