use strict;
use warnings;
use Inline C => <<'END_C';

void
test1(SV *sv)
{
    STRLEN len;
    char *ptr = SvPVutf8(sv, len);
    warn("%s\n", ptr);
}
void
test2(SV *sv)
{
    STRLEN len;
    char *ptr = SvPVutf8(sv, len);
    warn("%s\n", ptr);
    SvPOK_off(sv);
}

END_C

warn "SvPVutf8 messes up the behavior of \$1.\n";
my $foo = "a b c";
while ( $foo =~ /(\w)/g ) {
    warn "$1\n";
    test1($1);
}

warn "Invoking SvPOK_off solves the problem.\n";
while ( $foo =~ /(\w)/g ) {
    warn "$1\n";
    test2($1);
}