This sticks a newline between every line in the input file. You also get a blank line at the end, which strictly speaking, is not "between" two lines.

#perl linesBetween.pl linesBetweenTest.txt
#makes backup just in case
use strict;
use warnings;

my $infile = shift;

$infile =~ /(.*)(\.txt$)/ or die 'Input file must be of type .txt';

my $backupfile = "$1.bak";
my $tmpfile = "$1.tmp";
`copy $infile $1.bak`;

my $F;
my %lines;

open F, "< $infile" or die "couldn't open $infile";
open G, "> $tmpfile" or die "couldn't open $tmpfile";


while (<F>) {
    print G "$_\n";
}
close F;
close G;

`del $infile`;
`move $tmpfile $infile`;
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