Would it be satisfactory to simply ignore the first call?

Using the example Khen1950fx gave (and fixing the scoping issue with $s) you could do it like this:

#!/usr/bin/perl 

use strict;
use warnings;
use Tk;

my $top = MainWindow->new();
$top->title("scale tests");

my $f = $top->Frame(
    -relief => 'ridge', 
    -borderwidth => 3,
);

my $s;
$s = $top->Scale(
    "-orient" => "horizontal",
    "-length" => 300,
    "-from"   => 0,
    "-to"     => 250,
    "-tickinterval" => 25,
    "-command" => sub {
        use feature 'state';
        state $ncalls++ or return;
        printf "Callback: Scale(%d)\n", $_[0];
        $f->configure(
            '-width' => $s->get
        ) 
    }
);

$s->pack(-side => 'left',  -fill => 'y');
$f->pack(-side => 'right', -fill => 'y');

Tk::MainLoop;
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