# like this:

(my $newfile = $filename ) =~ s{.*[\\/]}{};
    # if there is no (back)slash, $newfile will be same as $filename

# or like this:

my @path = split /[\/\\]/, $filename;
my $newfile = pop @path;

# or use File::Basename, if that's appropriate to your needs

##</code><code>##

while(<$filename>) {

##</code><code>##

open( IN, $filename ) or die "$filename: $!";
while (<IN>) {