can't do it with Inline::C, can I?

Yes I can, no need in separate source file; here to prove my claims from parent node:

use strict;
use warnings;
use feature 'say';
use Benchmark 'cmpthese';
use Config;

say "$^V / $Config{ archname } / $Config{ gccversion }";

my $L = 1e6;
my $str = '1' x $L;

BEGIN {
our $c = << 'END_OF_C';
int %s( SV* str ) {
    STRLEN len;
    char* buf = SvPVbyte( str, len );
    len --;
    
    int acc = 0;
    int cum = 0;
    int max = -1;
    for( int i = 0; i < len; i ++ ) {
        int one = buf[ i ] - '0';
        acc += one;
        cum += one ? -1 : 1;
        if ( cum > max ) max = cum;
    }
    return acc + max + ( buf[ len ] == '1' );
}
END_OF_C
}

use Inline C => '
#pragma GCC target( "avx2" )
#pragma GCC optimize( "O3" )
' . sprintf our $c, 'mscore_optimised';

use Inline C => sprintf our $c, 'mscore_default';

my %tests = (
    default => sub { mscore_default( $str )},
    optimised => sub { mscore_optimised( $str )},
);

say 'uniform string:';
cmpthese -1, \%tests;

substr $str, rand $L, 1 , '0' for 1 .. $L;

say 'random string:';
cmpthese -1, \%tests;

__END__

v5.42.0 / MSWin32-x64-multi-thread / 13.2.0
uniform string:
            Rate   default optimised
default    897/s        --      -19%
optimised 1111/s       24%        --
random string:
           Rate optimised   default
optimised 245/s        --      -73%
default   897/s      266%        --
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