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    $f2 -> focus;

Switch: -state
... If the entry is readonly, then the value may not be changed using 
+widget commands and no insertion cursor will be displayed, even if th
+e input focus is in the widget; the contents of the widget may still 
+be selected. If the entry is disabled, the value may not be changed, 
+no insertion cursor will be displayed, the contents will not be selec
+table, ...

#!C:\Windows\Config\UMS_Scripts\perl\bin\perl
use strict;
...
    $ent_A->configure(-validatecommand => sub { first($ent_A, $ent_B) 
+});
    $ent_B->configure(-validatecommand => sub { second($ent_A, $ent_B)
+ });
}