Maybe the following will work for you

use strict;
use warnings;

use File::Copy;
use File::Find;
use Net::Netrc;
use Net::SFTP;
use MIME::Lite;
use Getopt::Std;
use Mail::Sender;

my $server = 'X.X.X.X';
my $user   = 'user';
my $sftp;
my %args = (
    ssh_args => {
        user => 'user',

        identity_files => ['/home/.ssh/id_rsa'],

        protocol => '2,1',

        debug => 1,

      }

);

my $linux      = "admin\@domain.net";
my $recipient1 = "recipient1\@domain.net";
my $recipient2 = "recipient2\@domain.net";
my $recipient3 = "recipient3\@domain.net>";

# write a log
BEGIN {
    use CGI::Carp qw(carpout);
    my $errorlog = "/srv/logs/transferlog.txt";
    open( LOG, ">$errorlog" ) or die("Unable to open $errorlog: $!\n")
+;
    print LOG "Errors:\n";
    carpout(*LOG);
}

# create backup subfolder
my @dt = localtime;
my $subfolder_name =
  ( ( ( ( 1900 + $dt[5] ) * 100 + 1 + $dt[4] ) * 100 + $dt[3] ) * 100 
++ $dt[2] )
  * 100 + $dt[1];
mkdir "/srv/OUT/$subfolder_name" or die "$subfolder_name: $!";

foreach my $from (</srv/*.txt>) {
    ( my $to = $from ) =~ s/.*\//\/HOME\//;

    # sftp file transfer
    $sftp = Net::SFTP->new( $server, %args )
      or die "could not open connection to $server\n";

    $sftp->put( $from, $to ) or die "could not upload $from\n";

    # move files to the backup directory
    unless ( move( "$from", "/srv/OUT/$subfolder_name" ) ) {
        print STDERR "Oops!  Couldn't move the file: $!";
    }

    move "/srv/logs/transferlog.txt", "/srv/OUT/$subfolder_name";

    sleep( 1 * 5 );
}

# send a mail if transfer completed
my $sender = new Mail::Sender { smtp => 'localhost', from => $linux };
$sender->MailFile(
    {
        to      => "$recipient1, $recipient2, $recipient3",
        subject => 'data transfer',
        msg     => 'data transfer should be completed',
        file    => "/srv/OUT/$subfolder_name/transferlog.txt"
    }
);

exit;
[download]

You might move setup of the sftp connection outside the loop. You should be able to transfer multiple files over a single connection.

The first time though the loop the log file will be moved but in subsequent iterations the move will fail because the log file is no longer in its original location.