How to traverse all the files in a directory; if it has subdirectories, I want to traverse files in subdirectories too

opendir(DIR,"$pwd") or die "Cannot open $pwd\n";
    my @files = readdir(DIR);
    closedir(DIR);
    foreach my $file (@files) {
        next if ($file !~ /\.txt$/i);
        my $mtime = (stat($file))[9];
        print $mtime;
        print "\n";
    }

Basically I want to note the timestamp of all the txt files in a directory. If there is a subdirectory I want to include files in that subdirectory too.

Can someone help me in modifying the above code so that it includes subdirectories too.

if i am using the code below in windows iam getting timestamps of all files which are in folders even outside my folder

 my @dirs = ("C:\\Users\\peter\\Desktop\\folder");
    my %seen;
    while (my $pwd = shift @dirs) {
            opendir(DIR,"$pwd") or die "Cannot open $pwd\n";
            my @files = readdir(DIR);
            closedir(DIR);
            #print @files;
            foreach my $file (@files) {
                    if (-d $file and !$seen{$file}) {
                            $seen{$file} = 1;
                            push @dirs, "$pwd/$file";
                    }
                    next if ($file !~ /\.txt$/i);
                    my $mtime = (stat("$pwd\$file"))[9];
                    print "$pwd $file $mtime";
                    print "\n";
            }
    }

use warnings;
use strict;

my @dirs = (".");
my %seen;
while (my $pwd = shift @dirs) {
        opendir(DIR,"$pwd") or die "Cannot open $pwd\n";
        my @files = readdir(DIR);
        closedir(DIR);
        foreach my $file (@files) {
                next if $file =~ /^\.\.?$/;
                my $path = "$pwd/$file";
                if (-d $path) {
                        next if $seen{$path};
                        $seen{$path} = 1;
                        push @dirs, $path;
                }
                next if ($path !~ /\.txt$/i);
                my $mtime = (stat($path))[9];
                print "$path $mtime\n";
        }
}