Move the match with /g into a condition of a while loop.

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";
}
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.