use Benchmark;

sub a {
    $_ = "this is a\ntest\n";
    s/(?:\s+|\n+)//gc;
}
sub b {
    $_ = "this is a\ntest\n";
    s/\s+//gc;
}
sub c {
    $_ = "this is a\ntest\n";
    tr/\n\r\t //d;
}

timethese(250000,{ a => \&a, b => \&b, c => \&c });


Benchmark: timing 250000 iterations of a, b, c...
         a:  5 wallclock secs ( 4.18 usr +  0.01 sys =  4.19 CPU) @ 59665.87/s (n=250000)
         b:  1 wallclock secs ( 1.61 usr +  0.03 sys =  1.64 CPU) @ 152439.02/s (n=250000)
         c:  1 wallclock secs ( 0.66 usr +  0.02 sys =  0.68 CPU) @ 367647.06/s (n=250000)