use strict;
use Benchmark qw< cmpthese >;
my $doc = ' "a string",' x 1_024_000;
cmpthese( -1, {
    copy => sub {
        return $1
            if  $doc =~ /(['"])/;
    },
    substr => sub {
        return substr( $doc, pos($doc)-1, 1 )
            if  $doc =~ /['"]/g;
    },
} );
__END__
           Rate    copy  substr
copy     38.6/s      --   -100%
substr 382480/s 991549%      --