use Benchmark qw[ cmpthese ];

my $str = '123456789';
cmpthese -1, {
 a=>q[ my ($a,$b) = $str =~  m/(23)[^8]+(8)/g; ], 
 b=>q[ my ($a,$b) = $str =~  m/(23)[^8]+(8)/; ],
 c=>q[ my ($a) = $str =~  m/(23)/g ],
 d=>q[ my ($a) = $str =~  m/(23)/; ],
};;