#!/usr/bin/perl
use strict;
use warnings;
# text should only match within these positions
my ($start_boundary, $end_boundary) = (11, 30);

my $regex = qr/hello world/i;

while (<DATA>) {

    # first method
    my $valid_data = substr($_, $start_boundary, $end_boundary - $start_boundary +1);
    if ( $valid_data =~ $regex ) {
            printf "1) %d %d '%s'\n",
                   $-[0],
                   $+[0],
                   substr($_, $-[0] + $start_boundary , $+[0] - $-[0]);
    }

    # second method
    if ( m/$regex/ ) {
        if ( $-[0] > $start_boundary && $+[0] < $end_boundary ) {
            printf "2) %d %d '%s'\n",
                   $-[0],
                   $+[0],
                   substr($_, $-[0], $+[0] - $-[0]);
        }
    }
}
__DATA__
some meaningful text containing "hello world" and more
hello world should not match here
in this row Hello World could match
also here my HELLO world has a chance
here hello world should be skipped