perl -p -0 -e 's/(?<!\n)\n\n(?!\n)/\n/g;' singleblank.txt

##</code><code>##

#!/usr/bin/perl -w
use strict;

# set input record separator $/ (see perldoc perlvar) to undef
# to "slurp" the file
my $t = do {local $/; (<DATA>)};

# (?<!\n) is negative lookbehind, no \n before
# (?!=\n) in negative lookahead, no \n after
# in between is \n\n - two consecutive newlines eq a single blank line
$t =~ s/(?<!\n)\n{2}(?!=\n)/\n/g;

print $t;
 
__DATA__
none
one

two


three



one

none
three



two


none
one


##</code><code>##

none
one
two

three


one
none
three


two

none
one