How would you code that with the perlapi

Possibly not what you're after, but I think *I* would actually do it as a callback:

use warnings;

use Inline C => <<'EOC';

void C_foo(char *a) {
     dSP;

     ENTER;
     SAVETMPS;
     PUSHMARK(SP);
     POPMARK;
     PUSHMARK(SP);
     XPUSHs(sv_2mortal(newSVpv(a, 0)));
     PUTBACK;
     call_pv("foo", G_DISCARD);
     FREETMPS;
     LEAVE;
}


EOC

my $string = "AAA:BBB";

C_foo($string);

sub foo {
$_[0] =~ /^([^:]+):([^:]+)$/;
print "$1 - $2\n";
}
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Cheers,
Rob