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.

In reply to Re: Is there a way around grep? by toolic
in thread Is there a way around grep? by mdegges

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