my $file = 'myfile.txt';
my $txt = do { local (@ARGV, $/) = $file; <> };

$txt =~ s/[\r\n]/ /g;
$txt =~ s/ /rand() < 0.5 ? q{ } : "\n"/eg;

open(my $fh, '>', $file) or die "$0: Can't open $file for write: $!";
print $fh $txt;
close $fh;