This could be further simplified by using the special variable $. (INPUT_LINE_NUMBER) instead of $line_count:

open(F, "test.htm");
    while(<F>) {
        my @a = $_ =~ m{(http://hosting/image\d+\.jpg)}g;
        for my $url (@a) {
            print "URL [ $url ] found on line ($.)\n";
        }
    }
close(F);
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