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

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

##</code><code>##

>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';
"
:

:

##</code><code>##

string=<a\n>
   no modifier: regex=/^a$\n/
     match => $ matches only boundary, \n matches newline
   [ ... ]
   m modifier (multi line mode): regex=/^a$\n/m
     match => $ matches only boundary, \n matches newline

##</code><code>##

>perl -wMstrict -le
"my $s = qq{a\n};
 print $s =~ /^a$\n/  ? '   ' : 'NO ', 'match';
 print $s =~ /^a$\n/m ? '   ' : 'NO ', 'match';
"
NO match
NO match

##</code><code>##

>perl -wMstrict -le
"my $s = qq{a\n};
 print $s =~ /^ a $ \n/x  ? '   ' : 'NO ', 'match';
 print $s =~ /^ a $ \n/xm ? '   ' : 'NO ', 'match';
"
   match
   match