The exact approach will depend upon the problem you;re trying to solve.

my $match = qr/Hello\s*World/;
while (<DATA>) {
    print if /$match/;
}
__DATA__
Hello          World
Hello World
Hello World                .
[download]

Optionally, you could copy the string to match and remove the spaces prior to the match, but whether or not such a destructive behavior is desireable again depends upon your needs.