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'