Your first three regex do not need the modifier /g. You are using anchors, so the /g modifier is useless here. If you leave it out, the work will be done faster...

Your second regex (one whitespace at the end of line/string) is redundant, because it is already included in the third regex (one or more whitespace(s) at the end of line/string).

You really want to remove all whitespaces from the end of line, but only one from the beginning?

$string =~ s/^\S+//;  # assumed to remove *all* whitespaces from the b
+eginning
$string =~ s/\S+$//;  # assumed to remove *all* whitespaces from the e
+nd

$string =~ s/^\s+|\s+$//g;  # /g needed here
[download]

Benchmark:

#!/usr/bin/perl
# vi:ts=4 sw=4 et:
use strict;
use warnings;

use Benchmark qw( cmpthese );

my $results = cmpthese(
    -3, {
        'r2/g' => sub {
            my $string = ( ' ' x 1000 ) . 'x' . ( ' ' . 1000 );
            $string =~ s/^\s+//g;
            $string =~ s/\s+$//g;
        },
        'r2' => sub {
            my $string = ( ' ' x 1000 ) . 'x' . ( ' ' . 1000 );
            $string =~ s/^\s+//;
            $string =~ s/\s+$//;
        },
        'r1' => sub {
            my $string = ( ' ' x 1000 ) . 'x' . ( ' ' . 1000 );
            $string =~ s/^\s+|\s+$//g;
        }
    }
);
[download]

Result:

         Rate   r1 r2/g   r2
r1   135605/s   -- -19% -25%
r2/g 168081/s  24%   --  -7%
r2   181494/s  34%   8%   --
[download]

If your text in $string is multilined, you should check perlre for the /m modifier.

update: fixed typo.