In this case, think of grep as a for loop. It loops through all 5 of the filenames in your array variable (@index_file_names). For each filename ($_), it compares against the $filename value. If they are equivalent, an internal counter is incremented. grep returns the number of matches. If at least one match occured, the if clause is executed.

use File::Basename;

#list of filenames to normalize
my @index_file_names=qw(index.html index.htm index.php index.asp index
+.cgi);

normalize_url('foo');
normalize_url('bar/index.php');

sub normalize_url {
    my $old_url = $_[0];
    chomp($old_url);
    #saves name at the end 
    my $filename=basename($old_url);

    if (grep {$_ eq $filename} @index_file_names) {
    #saves the directory part
    my $normalized_url=dirname($old_url);
    #$normalized_url;
    print "$old_url: found\n";
    }else{
        #don't need to normalize url
    #$old_url; 
    print "$old_url: not found\n";
    }
}

__END__

foo: not found
bar/index.php: found
[download]

This is a common use of grep.

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.