It is much better faster to do this in two steps: 

        $string =~ s/^\s+//;
        $string =~ s/\s+$//;

    Or more nicely written as: 

        for ($string) {
            s/^\s+//;
            s/\s+$//;
        }
[download]

    use Benchmark;

    timethese(500_000, {
            one => sub {
        my $foo = "    perl monks    ";
            $foo =~ s/^\s+|\s+$//g;
        },

        two => sub {
            my $foo = "    perl monks    ";
            for ($foo) {
                s/^\s+//;
                s/\s+$//;
            }
        },
    });
[download]

    Benchmark: timing 500000 iterations of one, two...
       one: 11 secs (11.75 usr  0.00 sys = 11.75 cpu)
       two:  8 secs ( 7.57 usr  0.00 sys =  7.57 cpu)
[download]