$pat = "m/foo/";
print "foo1\n" if "foo" =~ $pat;

$pat = m/foo/;
print "foo2\n" if "foo" =~ $pat;

$pat = qr/foo/;
print "foo3\n" if "foo" =~ m/$pat/;

##</code><code>##

$pat = ($_ =~ m/foo/);
print "foo2\n" if "foo" =~ $pat;