Why do I getthe following output?

#! perl -w
my $haystack = "abcdefghijklmnopqrstuvwxyz";

$haystack =~ /(abc)|(cde)|(fgh)|(ghi)|(ijk)|(pqr)|(rst)|(xyz)/g;
if ( $#-) {
    for (1..$#+) {
        print "match at $-[$_]\n"
    }
}
__OUTPUT__

C:\test>test
match at 0
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
Use of uninitialized value in concatenation (.) or string at C:\test\t
+est.pl line 7.
match at
[download]

Also, the docs suggests that ${$_} (within a string within the for loop above will return the matched part of the string?

From perlretut

    $x = "Mmm...donut, thought Homer";
    $x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
    foreach $expr (1..$#-) {
        print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr
+])\n";
    }
prints

    Match 1: 'Mmm' at position (0,3)
    Match 2: 'donut' at position (6,11)
[download]

How? Given $expr above is a number, how does "${1}" result in 'Mmm'?

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