#! perl -l

use strict;
use warnings;

my $s = join '', map {"$_\n"} qw(abc def ghi jkl);
my $m = qr/^def$/xms;

print "Match? \"$s\" =~ qr/$m/";
print "   1 : ", $s =~ $m ? "YES" : "NO?";
print "   2 : ", $s =~ qr/^def$/xms ? "YES" : "NO?";
print "   3 : ", $s =~ qr/def/xms ? "YES" : "NO?";

##</code><code>##

$ perl5.8.8 sq.pl
Match? "abc
def
ghi
jkl
" =~ qr/(?msx-i:^def$)/
   1 : NO?
   2 : NO?
   3 : YES