>perl -wMstrict -le
"my $s = qq{\n\n};
 print $s =~ /( \n $ \n )/x ? qq{:$1:} : 'no match';
"
:

:

>perl -wMstrict -le
"my $s = qq{\n\n};
 print $s =~ /( \n $ \n )/xm ? qq{:$1:} : 'no match';
"
:

: