while (<>) {
        ...             # your program goes here
    } continue {
        print or die "-p destination: $!\n";
    }


##</code><code>##

use strict;
use warnings;

my @files = @ARGV;

$^I = '.bak';

for my $file (@files) {
    local $/;
    @ARGV = $file;

    while (<>) {
        s/\n\n/\n/gs;
        print;
    }
}