s/$CR?$LF/\n/; 

##</code><code>##

my $CR="\015";
my $LF="\012";
my $foo="unknown_os\n";
$foo =~ s{$CR?$LF}{\n}xms;
print $foo;

##</code><code>##

? <-- HERE follows nothing in regex; marked by <-- HERE in m/
/ at -e line 4.