This appears to be an undocumented oddity of Tk::ProgressBar: when you give it a reference to a variable, it immediately stores the current value into the variable. I don't think widgets should do that, and certainly not without documenting it. But there you go.

To test this theory, add the following callback on another button:

sub dont_count_up
{
    my $progress = 0;
    $pb->configure( -variable => \$progress );
    print "progress=$progress\n";
}
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