#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use File::Find::Rule;

my @dirs = @ARGV ? @ARGV : ('.');
my $level = shift // 2;

my @files = File::Find::Rule->file()
    ->name('*.txt')
    ->maxdepth($level)
    ->in(@dirs);

print Dumper(\@files);

__END__

$ perl main.pl
$VAR1 = [
          'test.txt',
          'counts.txt',
          'SubFolder/foo.txt'
        ];