use Test::More;

sub trim { my $s = $_[0]; $s =~ s/^\s+(\S?.*\S)\s+$/$1/s; $s }

is( trim('  '), '' );
is( trim('a  '), 'a' );
is( trim('  a'), 'a' );
is( trim('  a  '), 'a' );
is( trim('ab  '), 'ab' );
is( trim('  ab'), 'ab' );
is( trim('  ab  '), 'ab' );
is( trim('a bb c  '), 'a bb c' );
is( trim('  a bb c'), 'a bb c' );
is( trim('  a bb c  '), 'a bb c' );

done_testing();

__END__
not ok 1
#   Failed test at ./t.pl line 12.
#          got: '  '
#     expected: ''
not ok 2
#   Failed test at ./t.pl line 13.
#          got: 'a  '
#     expected: 'a'
not ok 3
#   Failed test at ./t.pl line 14.
#          got: '  a'
#     expected: 'a'
ok 4
not ok 5
#   Failed test at ./t.pl line 16.
#          got: 'ab  '
#     expected: 'ab'
not ok 6
#   Failed test at ./t.pl line 17.
#          got: '  ab'
#     expected: 'ab'
ok 7
not ok 8
#   Failed test at ./t.pl line 19.
#          got: 'a bb c  '
#     expected: 'a bb c'
not ok 9
#   Failed test at ./t.pl line 20.
#          got: '  a bb c'
#     expected: 'a bb c'
ok 10
1..10
# Looks like you failed 7 tests of 10.
[download]

The one I could find with best benchmark and passes tests is s/^\s*((?:.*\S)?)\s*$/$1/s;, which is essentially like MRE_regx with + replaced with * (perhaps trizen typo-ed?)