As there are no capture groups in the regex (i.e. no parentheses), using any other index than 0 makes no sense. Under warnings, you'd even get lots of Use of uninitialized value in print.

Moreover, when there's no match, the @- and @+ arrays are empty, there's no -1 (that's what index does). You have to handle that case yourself.

This should work directly as shown and output the expected output:

#!/usr/bin/perl
use strict;
use warnings;

my $string = << "__EOS__";
foo bar baz
foo bar foo\tbaz ccc ddd
foo bar foo\tbaz ccc ddd\tfoo\tbar\tbaz
foo bar foo\t\tbaz ccc ddd\t\t\tfoo
__EOS__

open my $F0, '<', \$string or die $!;

while (<$F0>) {
    my $match;
    while (/\t+/g) {
        $match = 1;
        print  $-[0], ' ', $+[0], ' ';
    }
    print '-1 -1' unless $match;
    print "\n";
}
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