If you want to save the spawning time of a new shell, you can keep a shell open in a pipe, using IPC::Open3, and send your commands to it. Put the command to-check in a while loop

#!/usr/bin/perl 
use warnings;
use strict;
use IPC::Open3;

$|=1;

#my $pid=open3(\*IN,\*OUT,\*ERR,'/bin/bash');
my $pid=open3(\*IN,\*OUT,0,'/bin/bash');
# set \*ERR to 0 to send STDERR to STDOUT

my $cmd = 'date';

#send cmd to bash
print IN "$cmd\n";

my $result = <OUT>;
print $result;
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like you I thought about making it more efficient by reading the file driectly. What I did, was find an assembly version of "cat", and did something like

    sub refresh {
        my $pid = shift;
        #asmutils version of cat
    my @size = split "\n", `/home/zentara/perl5lib/cat /proc/$pid/stat
+us`;
    #my @size = split "\n", `cat /proc/$pid/status`;
    
        (my $vmsize) = grep { /VmSize/ } @size;
        my (undef, $size) = split '   ', $vmsize;
        $t->configure(-text => "PID: $pid ->  $size");
        if ($size eq '') { exit }
    }
}
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I never really accurately tested which one is fastest, but the assembly cat version made me feel fast. :-)