#! Perl -w
use strict;
use File::Find;

my $fdir='C:';
my @filelist = ();
find(\&wanted, $fdir);
write_file() if (@filelist);


sub write_file() {
.... # sub that writes @filelist to a text file
}

sub wanted {
    	my $thisfile = $File::Find::name;
    	push(@filelist, $thisfile) if ($thisfile =~ m/\.txt/); #for example

}