my $file; { local $/; $file = <>; }
$file =~ s/(?<!\n)\n(?!\n)//g
print $file;

##</code><code>##

perl script.pl infile > outfile

perl -i.bak script.pl file   # in-place

##</code><code>##

perl -0777pe's/(?<!\n)\n(?!\n)//g' infile > outfile

perl -i.bak -0777pe's/(?<!\n)\n(?!\n)//g' file