Using Net::OpenSSH::Parallel you can run all the operations in parallel. It is probably going to be much faster:

use Net::OpenSSH::Parallel;

my $pssh = Net::OpenSSH::Parallel->new;

my $machinelist;
my $machinelistfilename = "./machinelist.txt";
open($machinelist, "<", $machinelistfilename) or die $!;

while(<$machinelist>) {
  chomp;
  mkdir $_;
  -d $_ or die $!;
  $pssh->add_host($_, user => "username");
}

$pssh->push('*', scp_get => { glob => 1 }, '/tmp/nmonout/*', "./%HOST%
+/);
$pssh->push('*', command => "find /tmp/nmonout/ -mtime +2 |xargs rm");
$pssh->run;
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