I don't have any Macs so I tested on a PC.

I modified your code to get rid of the evals. With some luck, this may fix your (unidentified) problem.

#!/usr/bin/env perl
use strict;
use warnings;
use POSIX;
use Tk;

my %pid;
%pid=(
     DCDC_A_51=>{cmd=>'22 46 5B', cu=>'51', desc=>'DC/DC output curren
+t t30',     unit=>'A',         d=>2,  formula=>sub{sprintf("%.2f", U1
+6(1,4)/16)}},
     DCDC_V_51=>{cmd=>'22 46 5C', cu=>'51', desc=>'DC/DC output voltag
+e t30',     unit=>'V',         d=>2,  formula=>sub{sprintf("%.2f", U1
+6(25,253)/512)}, val=>'---'},
     DCDC_W_51=>{                           desc=>'DC/DC power output'
+,           unit=>'W',                 action=>sub{sprintf("%.0f", ($
+pid{DCDC_V_51}{val} * $pid{DCDC_A_51}{val}))}, val=>'---'}
);
my $mw=MainWindow->new();
$mw->Button(-text=>"   Test   ", -command=>sub{test()}, -font=>'Helvet
+ica 18 bold', -pady=>20)->pack();
MainLoop;

sub test {
    $pid{DCDC_V_51}{val}=$pid{DCDC_V_51}{formula}->(); print $pid{DCDC
+_V_51}{val} ."\n";
    $pid{DCDC_A_51}{val}=$pid{DCDC_A_51}{formula}->(); print $pid{DCDC
+_A_51}{val} ."\n";
    $pid{DCDC_W_51}{val}=$pid{DCDC_W_51}{action}->();
    print $pid{DCDC_W_51}{val} ."\n";
}

sub U16 {
    my($a,$b)=@_;
    return ($a*256+$b);
}
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