my  $nbridx   =  1;
    my  $filename;
    while( $filename = <DATA> ) {
        next if $filename =~ m/^\s*#/;
        chomp( $filename );

        # First split filename into alternating non-numeric and numeric
        # chunks.  Any defined string will match here.
        my  @a = $filename =~ m/ (\D+)? (\d+)? /xg;
        
        # The m//g always tries once too many, so throw away the last
        # two <undef> array elements.
        splice(@a,-2,2);
        
        # An empty string would result in an empty array.
        next  unless @a;
        # Leading/embedded/trailing spaces are not dealt with here.
        
        # Now we have an array where numeric chunks are at odd-numbered
        # indexes and non-numeric chunks are at even-numbered indexes.
        # Note that the leading and/or trailing elements may be undef.
        # Examples:
        #    'number.', '9'
        #    '<undef>', '1', '.cgi', '<undef>'
        #    'bottles', '99', '.ale', '<undef>'
      # printf "I split into '%s'\n", join("', '", doud(@a) );

        # Figure out how many number chunks exist in this array then
        # check against how many are needed.
        my  $nbrcnt = int( scalar(@a) / 2 );
            $nbrcnt--  unless defined $a[-1];
        if( $nbridx == 0  or  abs($nbridx) > $nbrcnt ) {
            printf "This filename only has %d number chunks and you asked for the %d'th one!\n",
                $nbrcnt, $nbridx;
            next;
        }
        # Now a little math to figure out the array index of wanted number
        my  $chkidx = ($nbridx>0) ? (  $nbridx            * 2 - 1 )
                                  : ( ($nbridx + $nbrcnt) * 2 + 1 );
      # printf "  Desired index %d    Number Chunks %d    Internal index %d\n",
      #                 $nbridx, $nbrcnt, $chkidx;

        # All the actual manipulation of $a[$chkidx] would be the
        # next little bit (cough) to implement here

        # Show the chunk we operated on and rebuild the string.
        $filename = join( '', grep { defined $_ } @a );
        printf "  Using '%s' from '%s'\n", $a[$chkidx], $filename;
    }
sub doud { map { defined($_) ? $_ : '<undef>' } @_ };

__END__

wrong_number!
01-file02.html
11-file22tom33bill.html
1.cgi
number.9
bottles99.ale
0bottles.beer
666

##</code><code>##

   my  @a = $filename =~ m/ (\D+)? (\d+)? /xg;