That routine is a little broken -- the return value of index() can be 0 on success, and you'll want to set $pos to the current position plus whatever the length of the separator is at each iteration.

One version of using index() that fares somewhat comparably to the m_while_M routine is:

my $_index = sub {
    my $cnt = 0;
    local $[ = 1;  # ook!
    my $pos = -1;
    ++$cnt while $pos = index($dta,"\n\n",$pos+2);
    $cnt{_index} = $cnt;
};
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But an alternate version of the m_while_M routine (using while as a statement modifier rather than the block form) seems to be the best I can come up with at this hour:

my $_while_re = sub{
    my $cnt = 0;
    my $recsep = "\n\n";
    $cnt++ while $dta =~ /$recsep/g;
    $cnt{_while_re} = $cnt;
};
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