You are thinking of 'file slurping' - which is done by modifying the $/ perl line separator (use $/ = undef; to slurp in one go), or using a module like File::Slurp.

Then you can use a regex to remove single newline characters, but leave alone double ones using a lookahead assertion, or something simple like:

use strict;
use warnings;

my $line = "foo\nbar\n\nbaz";
print "Before :\n \'$line\'\n";
#Before :
#'foo
#bar
#
#baz'

$line =~ s/\n([^\n])/$1/;
print "after : \'$line\'\n";
#after :
#'foobar
#
#baz'
[download]

HTH!